Top name, keen ultimately to enter female party (3,5)
I believe the answer is:
hen night
'female party' is the definition.
(party for a bride to be and her female friends)
'top name keen ultimately to enter' is the wordplay.
'top' becomes 'height' (synonyms).
'name' becomes 'n' (common abbreviation e.g. on forms).
'ultimately' suggests the final letters.
'to enter' means one lot of letters goes inside another.
The last letter of 'keen' is 'n'.
'n'+'n'='nn'
'height' placed around 'nn' is 'HEN NIGHT'.
(Other definitions for hen night that I've seen before include "Party for women only" , "prematch party" , "party of girls" , "then may the ladies rejoice" , "Pre-wedding celebration for ladies only" .)