Sea-bathing, mostly (2,3,4)
I believe the answer is:
in the main
'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" .)