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' is a reversal indicator.
'eg'+'dim'='egdim'
'egdim' reversed gives '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" .)