Feign problem in sea, swimming (6)
I believe the answer is:
assume
'feign' is the definition.
(I know that feign can be written as assume)
'problem in sea swimming' is the wordplay.
'problem' becomes 'sum' (arithmetic problem).
'in' is an insertion indicator.
'swimming' indicates an anagram (letters swim around into a new order).
'sea' anagrammed gives 'ase'.
'sum' going into 'ase' is 'ASSUME'.
(Other definitions for assume that I've seen before include "Take for granted or agreed" , "Adopt or take for granted" , "Wrongly take" , "Suppose; put on" , "Take as true without proof" .)