Don't tuck your shirt in for party! (3,2,3,4,3)
I believe the answer is:
let it all hang out
'don't tuck your' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'shirt in for party' is the wordplay.
I cannot quite understand how this works, but
'shirt' could be 't' (as in a T-shirt) and 't' is present in the answer.
'party' could be 'l' and 'l' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for let it all hang out that I've seen before include "hold nothing back?" , "What one should do on washday" , "Be completely uninhibited" , "Relax totally" , "finished the washing, perhaps" .)