Catch girl with boy's heart (5)
I believe the answer is:
lasso
'catch' is the definition.
(catch with a rope)
'girl with boy's heart' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'with' is a charade indicator (letters next to each other).
'heart' says to take the centre.
The middle letter of 'boy' is 'o'.
'lass'+'o'='LASSO'
(Other definitions for lasso that I've seen before include "Rope used for catching cattle" , "Cowboy's noosed rope" , "Rope for throwing" , "snare" , "Cowboy's rope with sliding noose" .)