To a doctor and sailor, it’s dull (4)
I believe the answer is:
drab
'dull' is the definition.
('drab' can be a synonym of 'dull')
'to a doctor and sailor' is the wordplay.
'to a doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor'**).
'and' is a charade indicator (letters next to each other).
'sailor' becomes 'ab' (abbreviation for able seaman).
'dr'+'ab'='DRAB'
'it's' is the link.
(Other definitions for drab that I've seen before include "Dingy, dull" , "Dull kind of bard" , "Dull light brown colour" , "Dull and colourless" , "Colourless kind of bard" .)