The most common and most likely answer to this crossword clue is the 4 plus 2 plus 2 letter word STEP IT UP
S
T
E
P
I
T
U
P
Buddy explains!
The straight part
We know that "Come on!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case come on! = step it up.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.