Serenade briefly interrupted by cheers after sailor's refrain (7)
I believe the answer is:
abstain
'refrain' is the definition.
(synonyms)
'serenade briefly interrupted by cheers after sailor's' is the wordplay.
'serenade briefly' becomes 'sin' (I am not sure about this - if you are sure you should believe this answer much more).
'interrupted by' is an insertion indicator.
'cheers' becomes 'ta' (both can informally mean thank you).
'after' says to put letters next to each other.
'sailor' becomes 'ab' (abbreviation for able seaman).
'sin' placed around 'ta' is 'stain'.
'stain' put after 'ab' is 'ABSTAIN'.
(Other definitions for abstain that I've seen before include "Don't drink" , "Resist" , "Refuse to imbibe" , "Avoid" , "decline at voting time" .)