Dim Coot left dope outside (6)
I believe the answer is:
twilit
'dim' is the definition.
(I've seen this before)
'coot left dope outside' is the wordplay.
'coot left' becomes 'li' (I am not sure about this - if you are sure you should believe this answer much more).
'dope' becomes 'twit' (both can mean a stupid person).
'outside' is an insertion indicator (some letters go outside others).
'li' put within 'twit' is 'TWILIT'.
Can you help me to learn more?
(Other definitions for twilit that I've seen before include "dusky?" , "Not being fully illuminated" , "semi-darkness" , "in the gloom" , "Gently illuminated" .)