Determined to form a group (3)
I believe the answer is:
set
I believe this clue is a triple definition!
'determined' is the first definition.
(I know that determined can be written as set)
'to form' is the second definition.
(thesaurus)
'a group' is the third definition.
(I know that set is a type of group)
(Other definitions for set that I've seen before include "Solidify like jelly" , "Fix in place; similar group" , "Prescribed" , "Scenery etc. for a drama" , "Mount (precious stone)" .)