Taking part in attempt to create threesome (7)
I believe the answer is:
trinity
'threesome' is the definition.
(synonyms)
'taking part in attempt' is the wordplay.
'taking part' becomes 'init' (someone involved in something is 'in it').
'in' indicates putting letters inside.
'attempt' becomes 'try' (synonyms).
'init' put into 'try' is 'TRINITY'.
'to create' acts as a link.
(Other definitions for trinity that I've seen before include "Term" , "Group of three considered as one" , "God" , "House" , "Cambridge college founded by Henry VIII" .)