Suppose it's a problem, in a rough sea (6)
I believe the answer is:
assume
'suppose' is the definition.
(assume can mean to suppose or expect)
'a problem in a rough sea' is the wordplay.
'a problem' becomes 'sum' (arithmetic problem).
'in' is an insertion indicator.
'a rough' indicates anagramming the letters (a rough or broken ordering of the letters).
'sea' anagrammed gives 'ase'.
'sum' inserted inside 'ase' is 'ASSUME'.
'it's' acts as a link.
(Other definitions for assume that I've seen before include "Dark brown fur" , "Suppose - appropriate - feign - adopt" , "Guess" , "Be arrogant" , "Take as read" .)