A rather rough sea, too (2,4)
I believe the answer is:
as well
'too' is the definition.
(synonyms)
'a rather rough sea' is the wordplay.
'rather rough sea' becomes 'swell' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'swell'='AS WELL'
Can you help me to learn more?
(Other definitions for as well that I've seen before include "To boot" , "on top" , "Appropriate" , "A swell sort of addition" , "with equal effect" .)