Old record is guarded by unreasonable audio specialist (9)
I believe the answer is:
otologist
'specialist' is the definition.
(otologist is a kind of specialist)
'old record is guarded by unreasonable audio' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'record' becomes 'log' (logging is a kind of recording).
'guarded by' indicates putting letters inside.
'unreasonable audio' becomes 'tot' (I can't justify this - if you can you should give a lot more credence to this answer).
'log'+'is'='logis'
'logis' placed within 'tot' is 'tologist'.
'o'+'tologist'='OTOLOGIST'
Can you help me to learn more?
(Another definition for otologist that I've seen is " Ear specialist".)