Cheerless Celtic poet sent up (4)
I believe the answer is:
drab
'cheerless' is the definition.
(I've seen this before)
'celtic poet sent up' is the wordplay.
'celtic poet' becomes 'bard' (I've seen this before).
'sent up' shows that the letters should be reversed in order.
'bard' reversed gives 'DRAB'.
(Other definitions for drab that I've seen before include "Gloomy" , "Cheerless, shabby" , "Dowdy" , "Shabby; a slattern (arch.)" , "Dull, uninteresting" .)