Moon about end of party, to act audaciously (3,2,2)
I believe the answer is:
try it on
'act audaciously' is the definition.
'moon about end of party' is the wordplay.
'moon' becomes 'triton' (I've seen this before**).
'about' is an insertion indicator.
'end of' says to take the final letters.
The last letter of 'party' is 'y'.
'triton' placed around 'y' is 'TRY IT ON'.
'to' acts as a link.
(Other definitions for try it on that I've seen before include "Push one's luck" , "Deliberately test someone's patience (colloq.)" , "Before buying a dress, you should" , "Test the boundaries" , "Attempt a fitting" .)