Stringed instrument mostly good for Scottish sailor (6)
I believe the answer is:
guitar
'stringed instrument' is the definition.
(I know that guitar is a type of stringed instrument)
'mostly good for scottish sailor' is the wordplay.
'mostly' means to remove the last letter.
'good for scottish' becomes 'guid' (Scottish dialect form of 'good').
'sailor' becomes 'tar' (slang term for a sailor).
'guid' with its last letter taken off is 'gui'.
'gui'+'tar'='GUITAR'
(Other definitions for guitar that I've seen before include "Six- or 12-stringed instrument" , "Stringed instrument often played by pop groups" , "Talk under one's breath" , "13's lute" , "Popular stringed instrument used by Beatles" .)