Instinctive swimming rivals, extremely competitive at heart (8)
I believe the answer is:
visceral
'instinctive' is the definition.
(I've seen this before)
'swimming rivals extremely competitive at heart' is the wordplay.
'swimming' is an anagram indicator.
'extremely' means to remove the middle letters.
'at heart' means one lot of letters goes inside another.
'competitive' with its middle removed is 'ce'.
'rivals' anagrammed gives 'visral'.
'visral' placed around 'ce' is 'VISCERAL'.
(Other definitions for visceral that I've seen before include "Relating to basic human instincts rather than the intellect" , "Relating to deep inner feelings" , "Relating to internal organs of the body" , "Relating to gut reaction" , "Appertaining to the guts" .)