Bar from first sea shanty (4)
I believe the answer is:
shut
'bar' is the definition.
(thesaurus)
'first sea shanty' is the wordplay.
'first' indicates taking the first letters.
'shanty' becomes 'hut' ('hut' can be a synonym of 'shanty').
The initial letter of 'sea' is 's'.
's'+'hut'='SHUT'
'from' acts as a link.
(Other definitions for shut that I've seen before include "Slam" , "Push to" , "Out of business" , "Draw to" , "Close" .)