Sailor, born in prison, leaves? (7)
I believe the answer is:
cabbage
'leaves?' is the definition.
(leafy vegetable)
'sailor born in prison' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'born' becomes 'b' (abbreviation next to the year of someone's birth).
'in' indicates putting letters inside.
'prison' becomes 'cage' (cage is a kind of prison).
'ab'+'b'='abb'
'abb' put inside 'cage' is 'CABBAGE'.
(Other definitions for cabbage that I've seen before include "Vegetable with dense head of leaves" , "Green vegetable" , "Kind of 13" , "A Brassica plant" , "Veg" .)