Gibbon? Surprisingly, it's a rhino (9)
I believe the answer is:
historian
'gibbon?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'surprisingly it's a rhino' is the wordplay.
'surprisingly' indicates an anagram (a surprising ordering of the letters).
'its'+'a'+'rhino'='itsarhino'
'itsarhino' with letters rearranged gives 'HISTORIAN'.
Can you help me to learn more?
(Other definitions for historian that I've seen before include "Past master" , "So thin air suits recorder and analyst of past events" , "One sort of academic" , "student of the past" , "Writer about the past" .)