Swim round about and respire (7)
I believe the answer is:
breathe
'respire' is the definition.
(synonyms)
'swim round about' is the wordplay.
'swim' becomes 'bathe' (bathe is a kind of swim).
'round' means one lot of letters goes inside another.
'about' becomes 're' (common abbreviation for 'with reference to').
'bathe' enclosing 're' is 'BREATHE'.
'and' is the link.
(Other definitions for breathe that I've seen before include "Absorb air" , "Inhale/exhale" , "sigh" , "Take into mouth" , "Do this before you go under" .)