Mostly, then, I swim, being by the sea (2,3,4)
I believe the answer is:
in the main
'mostly' is the definition.
(I've seen this before)
'then i swim being by the sea' is the wordplay.
'swim' indicates an anagram.
'being by' means one lot of letters go next to another.
'the sea' becomes 'main' ('main' can be a synonym of 'sea'**).
'then'+'i'='theni'
'theni' with letters rearranged gives 'inthe'.
'inthe'+'main'='IN THE MAIN'
(Other definitions for in the main that I've seen before include "Primarily" , "nautically?" , "where to find water" , "Almost entirely" , "seaborne?" .)