How old prisoner is transported, when one can agree to act (3,2,7)
I believe the answer is:
age of consent
'when one can agree to act' is the definition.
(when someone can consent to an act)
'how old prisoner is transported' is the wordplay.
I cannot really understand how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found in the answer.
'prisoner' could be 'con' (abbreviation for 'convict') and 'con' is found within the answer.
'transported' could be 'sent' ('send' can be a synonym of 'transport') and 'sent' is present in the answer.
This explanation may well be incorrect...
(Other definitions for age of consent that I've seen before include "Beyond which relations allowed" , "Time when agreement becomes legally valid" , "Legal majority?" .)