Dogs retain a passion ultimately for sports (7)
I believe the answer is:
mutants
'sports' is the definition.
('mutant' can be a synonym of 'sport')
'dogs retain a passion ultimately' is the wordplay.
'dogs' becomes 'mutts' (mutt is a kind of dog**).
'retain' means one lot of letters goes inside another.
'ultimately' says to take the final letters.
The last letter of 'passion' is 'n'.
'a'+'n'='an'
'mutts' going around 'an' is 'MUTANTS'.
'for' acts as a link.
(Other definitions for mutants that I've seen before include "organism's genetic variations" , "freaks" , "Organisms with gene alterations" .)