Genius, master, incredibly old (7)
I believe the answer is:
maestro
'genius' is the definition.
(I've seen this before)
'master incredibly old' is the wordplay.
'incredibly' indicates an anagram.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'master' is an anagram of 'maestr'.
'maestr'+'o'='MAESTRO'
(Other definitions for maestro that I've seen before include "Means of payment" , "Orchestral conductor" , "Artist of consummate skill: conductor" , "Top player" , "Virtuoso" .)