Plane crash reaches agreement on a penitentiary (5,10)
I believe the answer is:
penal settlement
'a penitentiary' is the definition.
(thesaurus)
'plane crash reaches agreement' is the wordplay.
I cannot quite see how this works, but
an anagram of 'plane' is 'penal' which is within the answer.
'agreement' could be 'settlement' (settlement is a kind of agreement) and 'settlement' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'on' is the link.
Can you help me to learn more?
(Another definition for penal settlement that I've seen is " place housing convicts".)