Greens - a group once almost locked up? (7)
I believe the answer is:
cabbage
'greens' is the definition.
(thesaurus)
'a group once almost locked up?' is the wordplay.
'a group once' becomes 'abba' (70s and 80s pop group).
'almost' means to remove the first letter.
'locked' is an insertion indicator (I've seen this in other clues).
'up?' becomes 'cage' (I am not sure about this - if you are sure you should believe this answer much more).
'abba' with its first letter taken away is 'bba'.
'bba' going within 'cage' is 'CABBAGE'.
(Other definitions for cabbage that I've seen before include "Beg a cab to take the vegetable" , "Main ingredient of sauerkraut" , "A Brassica plant" , "Vegetable with dense head of leaves" , "We bag ice bath (anag) -- butterfly" .)