One including learner that's not illuminated (5)
I believe the answer is:
unlit
'not illuminated' is the definition.
(I've seen this before)
'one including learner' is the wordplay.
'one' becomes 'unit' (synonyms).
'including' means one lot of letters goes inside another.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'unit' enclosing 'l' is 'UNLIT'.
'that's' is the link.
(Other definitions for unlit that I've seen before include "such as dark alley?" , "Perhaps invisible" , "not provided with a lamp?" , "Without illumination" , "Left in the dark" .)