Gloomy, except for returning by day (4)
I believe the answer is:
drab
'gloomy' is the definition.
(I've seen this before)
'except for returning by day' is the wordplay.
'except for' becomes 'bar' (I've seen this before).
'returning' says the letters should be written in reverse.
'by' says to put letters next to each other.
'day' becomes 'd'.
'bar' backwards is 'rab'.
'rab' after 'd' is 'DRAB'.
(Other definitions for drab that I've seen before include "Drearily dull; slattern" , "Lacking brightness or colour" , "Dingy - shabby" , "kind of tart" , "Cheerless, colourless" .)