Learner in military group not exactly bright (5)
I believe the answer is:
unlit
'not exactly bright' is the definition.
(I've seen this before)
'learner in military group' is the wordplay.
'learner' becomes 'l'.
'in' means one lot of letters goes inside another.
'military group' becomes 'unit' (I've seen this before).
'l' placed inside 'unit' is 'UNLIT'.
(Other definitions for unlit that I've seen before include "dim" , "Shadowy" , "Not illuminated" , "Lacking illumination" , "Not yet fired up" .)