Swimmer in buff coming up with shark's heart (4)
I believe the answer is:
tuna
'swimmer' is the definition.
(tuna is a type of fish)
'buff coming up with shark's heart' is the wordplay.
'buff' becomes 'nut' (both can mean an enthusiast or fan).
'coming up' is a reversal indicator.
'with' means one lot of letters go next to another.
'heart' says to take the centre.
The centre of 'shark' is 'a'.
'nut' written backwards gives 'tun'.
'tun'+'a'='TUNA'
'in' is the link.
(Other definitions for tuna that I've seen before include "Marine food fish" , "Yellow-finned fish" , "Large tasty fish, fond of a nut" , "Large fish found in a nut" , "Widely-eaten fish" .)