Sailor picked up in ship not half dull (7)
I believe the answer is:
tarnish
'dull' is the definition.
(I've seen this before)
'sailor picked up in ship not half' is the wordplay.
'sailor' becomes 'tar' (term for a sailor).
'picked up' is a reversal indicator.
'not half' suggests halving the letters.
'in' written backwards gives 'ni'.
'ship' with half the letters taken is 'sh'.
'tar'+'ni'+'sh'='TARNISH'
(Other definitions for tarnish that I've seen before include "Cause to lose lustre, silver or reputation" , "Cause to lose lustre, say silver" , "Film" , "Taint" , "Mountain lake" .)