Lacking lustre a sailor is under the doctor (4)
I believe the answer is:
drab
'lacking lustre a' is the definition.
I can't tell whether this definition defines the answer.
'sailor is under the doctor' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'is under' means one lot of letters go next to another.
'the doctor' becomes 'dr' (abbreviation).
'ab' after 'dr' is 'DRAB'.
Can you help me to learn more?
(Other definitions for drab that I've seen before include "Shabby; a slattern (arch.)" , "kind of tart" , "Gloomy" , "Dull and boring" , "Grey or light brown cloth" .)