The most common and most likely answer to this crossword clue is the 2 letter word DO
I've seen this crossword clue in The Daily Telegraph
D
O
More clues leading to the result DO
- Party sentiment upset left wing social reformer?
- During parties one's up for it?
- Work load then left for me?
- Philanthropist, setter, say, rounds on upset friend now and then
- Desperately determined in entrance and exit
- Helpful types needing Her Majesty in party merchandise
- Method of carrying out home improvements? Study loo fire damaged
- Practical type working tediously for queen
- One helping animal needing to get round by foreign river
- Entrance by duke that is doggedly determined
Buddy explains!

The straight part
We know that "show tune" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case show tune = do.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DO mean?
DO noun- A party, celebration, social function.
- A hairdo.
- Something that can or should be done (usually in the phrase dos and don'ts).
- A deed; an act.
- Ado; bustle; stir; to-do; A period of confusion or argument.
- A cheat; a swindler.
- An act of swindling; a fraud or deception.
- (auxiliary) A syntactic marker.
- To perform; to execute.
- To cause, make (someone) (do something).
- To suffice.
- To be reasonable or acceptable.
- (ditransitive) To have (as an effect).
- To fare, perform (well or poorly).
- (chiefly in questions) To have as one's job.
- To perform the tasks or actions associated with (something).
- To cook.
- To travel in, to tour, to make a circuit of.
- To treat in a certain way.
- To work for or on, by way of caring for, looking after, preparing, cleaning, keeping in order, etc.
- To act or behave in a certain manner; to conduct oneself.
- To spend (time) in jail. (See also do time)
- To impersonate or depict.
- (with 'a' and the name of a person, place, event, etc.) To copy or emulate the actions or behaviour that is associated with the person or thing mentioned.
- To kill.
- To deal with for good and all; to finish up; to undo; to ruin; to do for.
- To punish for a misdemeanor.
- To have sex with. (See also do it)
- To cheat or swindle.
- To convert into a certain form; especially, to translate.
- To finish.
- To work as a domestic servant (with for).
- (auxiliary) Used to form the present progressive of verbs.
- To cash or to advance money for, as a bill or note.
- (ditransitive) To make or provide.
- To injure (one's own body part).
- To take drugs.
- (in the form be doing [somewhere]) To exist with a purpose or for a reason.
- A syllable used in solfège to represent the first and eighth tonic of a major scale.
- As said before, likewise.