The most common and most likely answer to this crossword clue is the 5 letter word SPOUT
I've seen this crossword clue in Independent
S
P
O
U
T
Buddy explains!

The straight part
We know that "Go on!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case go on! = spout.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 SPOUT mean?
SPOUT noun- A tube or lip through which liquid or steam is poured or discharged.
- A stream of liquid.
- The mixture of air and water thrown up from the blowhole of a whale.
- To gush forth in a jet or stream
- To eject water or liquid in a jet.
- To speak tediously or pompously.
- To utter magniloquently; to recite in an oratorical or pompous manner.
- To pawn; to pledge.