Insect say with dark back (5)
I believe the answer is:
midge
'insect' is the definition.
(midge is a kind of insect)
'say with dark back' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'with' is a charade indicator (letters next to each other).
'dark' becomes 'dim' (similar in meaning).
'back' says the letters should be written in reverse.
'eg'+'dim'='egdim'
'egdim' written backwards gives 'MIDGE'.
(Other definitions for midge that I've seen before include "Small swarming biting insect" , "Minute mosquito-like fly" , "Be an obstacle" , "Two-winged fly" , "Small person" .)