Swimmer ultimately recognising mature following is hot! (8)
I believe the answer is:
goldfish
'swimmer' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'ultimately recognising mature following is hot' is the wordplay.
'ultimately' suggests the final letters.
'mature' becomes 'old' (associated in meaning).
'following' becomes 'f' (used when citing page numbers etc).
'hot' becomes 'h' (abbreviation eg on taps).
The final letter of 'recognising' is 'g'.
'g'+'old'+'f'+'is'+'h'='GOLDFISH'
Can you help me to learn more?
(Other definitions for goldfish that I've seen before include "Small orange carp" , "Orange-red freshwater fish" , "Small carp kept as pet" , "Pond or aquarium fish" , "Ornamental reddish Chinese carp (8)" .)