Doctor Jack is dull (4)
I believe the answer is:
drab
'dull' is the definition.
(similar in meaning)
'doctor jack' is the wordplay.
'doctor' becomes 'dr' (abbreviation).
'jack' becomes 'ab' (both can mean a sailor).
'dr'+'ab'='DRAB'
'is' is the link.
(Other definitions for drab that I've seen before include "Tedious" , "Gloomy" , "Dingy - shabby" , "Colourless kind of bard" , "Cheerless, shabby" .)