Crossword Genius

Mostly, then, I swim, being by the sea (2,3,4)

Ross

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?" .)

I've seen this clue in the Evening Standard.
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