Former prisoner leaves firm to make stools (7)
I believe the answer is:
excrete
'to make stools' is the definition.
(release excrement)
'former prisoner leaves' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'prisoner leaves' becomes 'crete' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ex'+'crete'='EXCRETE'
'firm' is the link.
I am not very happy about this link. It may belong to another bit of the clue.
(Other definitions for excrete that I've seen before include "Expel (waste) from the body" , "Defecate" , "Eliminate (from the body)" .)