Dim idiot swallowing lithium (6)
I believe the answer is:
twilit
'dim' is the definition.
(I've seen this before)
'idiot swallowing lithium' is the wordplay.
'idiot' becomes 'twit' (both can mean a stupid person).
'swallowing' means one lot of letters goes inside another.
'lithium' becomes 'Li' (Li is the chemical symbol for lithium).
'twit' going around 'li' is 'TWILIT'.
(Other definitions for twilit that I've seen before include "Dimly illuminated" , "Somewhat dim" , "Not being fully illuminated" , "in the gloom" , "in the dusk" .)