Absolute fan of jazz, say, or disco occasionally (9)
I believe the answer is:
categoric
'absolute' is the definition.
(I've seen this before)
'fan of jazz say or disco occasionally' is the wordplay.
'fan of jazz' becomes 'cat'.
'say' becomes 'eg'.
'occasionally' means one should take alternating letters.
The alternating letters of 'disco' are 'ic'.
'cat'+'eg'+'or'+'ic'='CATEGORIC'
(Other definitions for categoric that I've seen before include "Unqualified" , "Unambiguously explicit" , "Unconditional" , "express" , "Positive" .)