Dim learner in assembly (5)
I believe the answer is:
unlit
'dim' is the definition.
(I've seen this in another clue)
'learner in assembly' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'in' is an insertion indicator.
'assembly' becomes 'unit' (a group of people).
'l' put inside 'unit' is 'UNLIT'.
(Other definitions for unlit that I've seen before include "Left in the dark" , "such as dark alley?" , "Not yet burning" , "Until (anag.)" , "not switched on" .)