Crossword Genius

Sea-bathing, mostly (2,3,4)

Ross

I believe the answer is:

in the main

I'm a little stuck... Click here to teach me more about this clue!

'mostly' is the definition.
(I've seen this before)

'sea-bathing' is the wordplay.
I cannot really see how this works, but
'sea' could be 'main' (synonyms) and 'main' is found in the answer.
The remaining letters 'inthe' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for in the main that I've seen before include "seaborne?" , "Swimming" , "Frankfurters may be dipped here" , "Almost entirely" , "All things considered" .)

I've seen this clue in the Evening Standard.
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