An insect, say, not very bright, ascending (5)
I believe the answer is:
midge
'an insect' is the definition.
(midge is a kind of insect)
'say not very bright ascending' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'not very bright' becomes 'dim' (dim means poorly lit).
'ascending' says the letters should be written in reverse.
'eg'+'dim'='egdim'
'egdim' in reverse letter order is 'MIDGE'.
(Other definitions for midge that I've seen before include "Minute mosquito-like fly" , "Small person" , "Small biting fly" , "Tiny biting fly" , "Small swarming biting insect" .)