Prisoner finds doctor with old bird (6)
I believe the answer is:
condor
'bird' is the definition.
(condor is a kind of bird)
'prisoner finds doctor with old' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'finds' means one lot of letters go next to another.
'doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor'**).
'with' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'dr' placed around 'o' is 'dor'.
'con'+'dor'='CONDOR'
(Other definitions for condor that I've seen before include "Large bird of prey" , "Andean bird" , "Large vulture of S America" , "Largest New World vulture" , "Andean vulture" .)