Brownish colour very good in ocean (5)
I believe the answer is:
sepia
'brownish colour' is the definition.
(I've seen this before)
'good in ocean' is the wordplay.
'good' becomes 'pi' (old-fashioned term for pious).
'in' means one lot of letters goes inside another.
'ocean' becomes 'sea' (synonyms).
'pi' put within 'sea' is 'SEPIA'.
'very' acts as a link.
(Other definitions for sepia that I've seen before include "Brownish tone associated with old photographs" , "Rich reddish-brown colour" , "cuttle" , "Reddish-brown colour as in old photos" , "Brown of old photos" .)