Transient may be old enough to accept advance after prison sentence (4,2,7)
I believe the answer is:
bird of passage
'transient' is the definition.
(bird of passage can mean a transient person)
'old enough to accept advance after prison sentence' is the wordplay.
'old enough' becomes 'of age' (having come of age).
'to accept' means one lot of letters goes inside another.
'advance' becomes 'pass' (passing is a kind of advancing).
'after' is a charade indicator (letters next to each other).
'prison sentence' becomes 'bird' (to 'do bird' is to serve a prison sentence).
'ofage' enclosing 'pass' is 'ofpassage'.
'ofpassage' after 'bird' is 'BIRD OF PASSAGE'.
'may be' is the link.
(Other definitions for bird of passage that I've seen before include "transient" , "Swift, perhaps" , "Migrant - wanderer" , "One who is just passing through" , "Migratory flyer" .)