Pam forced prisoner to hide one part of stereo (9)
I believe the answer is:
amplifier
'part of stereo' is the definition.
(I know that amplifier is part of STEREO)
'pam forced prisoner to hide one' is the wordplay.
'forced' indicates anagramming the letters.
'prisoner' becomes 'lifer' (prisoner serving a life sentence).
'to hide' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'lifer' placed around 'i' is 'lifier'.
'pam' anagrammed gives 'amp'.
'amp'+'lifier'='AMPLIFIER'
(Other definitions for amplifier that I've seen before include "Device that strengthens" , "Sounds all up with this" , "Device augmenting sound" , "Device to boost sound levels" , "Electronic equipment to increase signal strength" .)