Gloomy linesman sent back (4)
I believe the answer is:
drab
'gloomy' is the definition.
(I know that gloomy can be written as drab)
'linesman sent back' is the wordplay.
'linesman' becomes 'bard' (bard is a kind of poet).
'sent back' says the letters should be written backwards.
'bard' reversed gives 'DRAB'.
(Other definitions for drab that I've seen before include "Lacklustre" , "Dreary and colourless" , "Dull sort of bard" , "Rather dull" , "Of a dull greenish-brown colour" .)