Gallons given to seaman in bars (7)
I believe the answer is:
grating
'bars' is the definition.
(I've seen this before)
'gallons given to seaman' is the wordplay.
'gallons' becomes 'G' (abbreviation).
'given to' says to put letters next to each other.
'seaman' becomes 'rating' (British term for a naval enlisted man).
'g'+'rating'='GRATING'
'in' is the link.
(Other definitions for grating that I've seen before include "Harsh - framework of crossed bars" , "Jarring" , "Frame of iron bars to block passage but allow air" , "Rasping - grille" , "Irritating - framework of bars" .)