Swimmer’s temperature taken internally by medic (8)
I believe the answer is:
sturgeon
'swimmer's' is the definition.
(type of fish)
'temperature taken internally by medic' is the wordplay.
'temperature' becomes 't' (abbreviation).
'taken internally by' indicates putting letters inside.
'medic' becomes 'surgeon' (I've seen this before).
't' going within 'surgeon' is 'STURGEON'.
(Other definitions for sturgeon that I've seen before include "Caviare-producing fish" , "Egg producer" , "caviar suppliers" , "Our gents caught a large fish" , "Fish sought for caviar" .)