Male gathering old maestro and commoner up, creative (8,5)
I believe the answer is:
bachelor party
'male gathering' is the definition.
(gathering for men only)
'old maestro and commoner up creative' is the wordplay.
'old maestro' becomes 'bach' (Johannes Sebastian Bach, 18th-century composer).
'and' means one lot of letters go next to another.
'commoner' becomes 'prole' (prole is a kind of commoner).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'creative' becomes 'arty' (I've seen this before).
'prole' reversed gives 'elorp'.
'bach'+'elorp'+'arty'='BACHELOR PARTY'
(Another definition for bachelor party that I've seen is " Men only get-together".)