At sea, smoker is mostly annoying (7)
I believe the answer is:
irksome
'annoying' is the definition.
(I've seen this before)
'at sea smoker is mostly' is the wordplay.
'at sea' is an anagram indicator.
'mostly' means to remove the first letter.
'smoker'+'is'='smokeris'
'smokeris' with its initial letter taken away is 'mokeris'.
'mokeris' anagrammed gives 'IRKSOME'.
(Other definitions for irksome that I've seen before include "Annoying or tiresome" , "Tediously annoying" , "Troublesome, vexing" , "Tiresome, irritating" , "'Annoying, irritating or boring (7)'" .)