Prisoner I’m surprised turned on old coach (7)
I believe the answer is:
hostage
'prisoner' is the definition.
(someone taken as a prisoner)
'i'm surprised turned on old coach' is the wordplay.
'i'm surprised' becomes 'oh' (expression of surprise).
'turned' says the letters should be written backwards.
'on' means one lot of letters go next to another.
'old coach' becomes 'stage' (I've seen this in another clue).
'oh' in reverse letter order is 'ho'.
'ho'+'stage'='HOSTAGE'
(Other definitions for hostage that I've seen before include "One held as security" , "Person taken as security" , "One held for money" , "Captive held for ransom" , "Prisoner held as surety" .)