Sailor breaks seat and calms down (6)
I believe the answer is:
abates
'calms down' is the definition.
(I've seen this in another clue)
'sailor breaks seat' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'breaks' is an anagram indicator.
'seat' anagrammed gives 'ates'.
'ab'+'ates'='ABATES'
'and' acts as a link.
(Other definitions for abates that I've seen before include "Becomes less intense" , "lowers" , "has a fall" , "Becomes less severe or strong" , "Calms down a" .)