Academic takes on the old expert in his field (5)
I believe the answer is:
doyen
'expert in his field' is the definition.
'doyen' can be an answer for 'expert' (doyen is a kind of expert). I'm unsure of the 'in his field' bit.
'academic takes on the old' is the wordplay.
'academic' becomes 'don' (type of academic - especially at Oxbridge).
'takes on' indicates putting letters inside.
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'don' enclosing 'ye' is 'DOYEN'.
Can you help me to learn more?
(Other definitions for doyen that I've seen before include "The most respected among experts" , "Respected senior member" , "As senior statesman" , "The oldest of the members" , "Most valued prof, eg" .)