Swimmer, tense, admitted by doctor (8)
I believe the answer is:
sturgeon
'swimmer' is the definition.
(sturgeon is a kind of fish)
'tense admitted by doctor' is the wordplay.
'tense' becomes 't' (abbreviation used in many dictionaries).
'admitted by' means one lot of letters goes inside another.
'doctor' becomes 'surgeon' (surgeon is a kind of doctor**).
't' going inside 'surgeon' is 'STURGEON'.
(Other definitions for sturgeon that I've seen before include "Large fish, valued for its roe" , "The fish that yields caviar" , "Caviar-producing fish" , "Our gents caught a large fish" , "Caviare-producing fish" .)