Gloomy sailor going to doctor (4)
I believe the answer is:
drab
'gloomy' is the definition.
(I've seen this before)
'sailor going to doctor' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'going to' says to put letters next to each other.
'doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor'**).
'ab' after 'dr' is 'DRAB'.
(Other definitions for drab that I've seen before include "Colourless like bard" , "Gloomy" , "Of a dull greenish-brown colour" , "Not cheering" , "Dowdy" .)