Historian, a big swinger (6)
I believe the answer is:
gibbon
'historian' is the definition.
(I've seen this before)
'a big swinger' is the wordplay.
I cannot really understand how this works, but
an anagram of 'big' is 'gib' which is present in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for gibbon that I've seen before include "Large tailless ape" , "A historian; SE Asian forest creature" , "Animal" , "Ape -- author of the 14 15 of the 5 7" , "author of Decline and Fall, etc" .)