Lacking brightness, poet is sent up (4)
I believe the answer is:
drab
'lacking brightness' is the definition.
(I've seen this before)
'poet is sent up' is the wordplay.
'poet' becomes 'bard' (bard is a kind of poet).
'is sent up' says the letters should be written backwards.
'bard' reversed gives 'DRAB'.
(Other definitions for drab that I've seen before include "Colourless kind of bard" , "Cheerless, shabby" , "Colourless like bard" , "Dismal < slattern" , "Dowdy" .)