Former prisoner having a part in prison escape, initially (7)
I believe the answer is:
parolee
'former prisoner' is the definition.
(prisoner who has been paroled)
'a part in prison escape initially' is the wordplay.
'part' becomes 'role' (eg a theatrical role).
'in' means one lot of letters goes inside another.
'initially' indicates taking the first letters.
The first letters of 'prison escape' is 'pe'.
'a'+'role'='arole'
'arole' going into 'pe' is 'PAROLEE'.
'having' is the link.
(Other definitions for parolee that I've seen before include "One is released on conditions" , "One conditionally released" , "Person no longer inside" .)