Have group sex, secretly at first (7)
I believe the answer is:
possess
'have' is the definition.
(I know that have can be written as possess)
'group sex secretly at first' is the wordplay.
'group' becomes 'posse' ().
'at first' suggests taking the first letters.
The initial letters of 'sex secretly' is 'ss'.
'posse'+'ss'='POSSESS'
(Other definitions for possess that I've seen before include "Have and hold" , "Own, keep" , "influence" , "Consume" , "Have - control" .)