Agreed sea tends to be rough (8)
I believe the answer is:
assented
'agreed' is the definition.
(assenting is a kind of agreeing)
'sea tends to be rough' is the wordplay.
'to be rough' indicates anagramming the letters.
'sea'+'tends'='seatends'
'seatends' is an anagram of 'ASSENTED'.
(Other definitions for assented that I've seen before include "Given agreement" , "Acceded." , "Agreed to do something" , "Expressed agreement" , "Acquiesced" .)