Plenty of big seas (6)
I believe the answer is:
oceans
This clue is a double definition.
'plenty' is the first definition.
(I've seen this in another clue)
'big seas' is the second definition.
(I've seen this before)
'of' is the link.
(Other definitions for oceans that I've seen before include "expanses?" , "Arctic and Indian" , "Vast bodies of water" , "Large amounts (of water)" , "Atlantic, Pacific etc" .)