Swimmer that’s about to go inside for a rest? (8)
I believe the answer is:
breather
'a rest?' is the definition.
(breather is a kind of rest)
'swimmer that's about to go inside' is the wordplay.
'swimmer' becomes 'bather' (synonyms).
'that's about to go' becomes 're' (I've seen this in other clues).
'inside' means one lot of letters goes inside another.
'bather' enclosing 're' is 'BREATHER'.
'for' is the link.
(Other definitions for breather that I've seen before include "lung or > liver?" , "Short pause for rest" , "Informally a pause for rest - or to take in air" , "A rest - lung?" , "state" .)