Sea, true, is rough and harsh (7)
I believe the answer is:
austere
'harsh' is the definition.
(thesaurus)
'sea true is rough' is the wordplay.
'is rough' is an anagram indicator.
'sea'+'true'='seatrue'
'seatrue' is an anagram of 'AUSTERE'.
'and' is the link.
(Other definitions for austere that I've seen before include "Abstemious, strict" , "Severe in appearance" , "Grim - USA tree (anag)" , "'Spartan, simple (7)'" , "Severe; lacking luxuries" .)