Learner in group lacking illumination (5)
I believe the answer is:
unlit
'lacking illumination' is the definition.
(I've seen this before)
'learner in group' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'in' means one lot of letters goes inside another.
'group' becomes 'unit' (eg a unit of soldiers).
'l' placed within 'unit' is 'UNLIT'.
(Other definitions for unlit that I've seen before include "not provided with a lamp?" , "not switched on" , "Not yet burning" , "Until (anag.)" , "Shady" .)