Old record is guarded by unreasonable audio specialist (9)

'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' (log is a kind of record).
'guarded by' means one lot of letters goes inside another.
'unreasonable audio' becomes 'tot' (I am not sure about this - if you are sure you should believe this answer much more).
'log'+'is'='logis'
'logis' placed within 'tot' is 'tologist'.
'o'+'tologist'='OTOLOGIST'

Can you help me to learn more?