Girls with large behinds, capturing his heart (7)
I believe the answer is:
lassies
'girls' is the definition.
(lassie is a kind of girl)
'large behinds capturing his heart' is the wordplay.
'large' becomes 'l'.
'behinds' becomes 'asses' ('ass' can be a synonym of 'behind'**).
'capturing' means one lot of letters goes inside another.
'heart' says to take the centre.
The central letter of 'his' is 'i'.
'asses' enclosing 'i' is 'assies'.
'l'+'assies'='LASSIES'
'with' is the link.
(Other definitions for lassies that I've seen before include "Young women" , "Wee Scottish girls" .)