It's dull for a doctor and a sailor (4)
I believe the answer is:
drab
'it's dull' is the definition.
('drab' can be a synonym of 'dull')
'a doctor and a sailor' is the wordplay.
'a doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor'**).
'and' means one lot of letters go next to another.
'a sailor' becomes 'ab' (abbreviation for able seaman).
'dr'+'ab'='DRAB'
'for' is the link.
(Other definitions for drab that I've seen before include "Dull-brown" , "Lacklustre" , "Dull, uninteresting" , "woman with slatternly habits" , "Colourless like bard" .)