Swimmer's jog around University (5)
I believe the answer is:
trout
'swimmer's' is the definition.
(trout is a kind of fish)
'jog around university' is the wordplay.
'jog' becomes 'trot' (synonyms).
'around' means one lot of letters goes inside another.
'university' becomes 'U' (abbreviation for university).
'trot' enclosing 'u' is 'TROUT'.
(Other definitions for trout that I've seen before include "Rainbow, say" , "old bag" , "bad-tempered woman" , "Rainbow, for example" , "Game fish related to salmon" .)