Amongst a dim assembly (4)
I believe the answer is:
amid
'amongst' is the definition.
(I've seen this in another clue)
'a dim assembly' is the wordplay.
'assembly' indicates anagramming the letters.
'dim' with letters rearranged gives 'mid'.
'a'+'mid'='AMID'
(Other definitions for amid that I've seen before include "Within" , "Among" , "'See . . . . the winter's snow' (carol)" , "Maid (anag)" , "In the centre of" .)