Young girl's story about a ship (6)
I believe the answer is:
lassie
'young girl's' is the definition.
('lassie' can be a synonym of 'young girl')
'story about a ship' is the wordplay.
'story' becomes 'lie' (I've seen this before).
'about' is an insertion indicator.
'ship' becomes 'ss'.
'a'+'ss'='ass'
'lie' enclosing 'ass' is 'LASSIE'.
(Other definitions for lassie that I've seen before include "Cinema dog who went home" , "TV series/film dog" , "dog in pictures" , "Canine film star" , "Screen canine" .)