A girl going back to get a boy (5)
I believe the answer is:
allen
'boy' is the definition.
'a girl going back to get a' is the wordplay.
'a girl' becomes 'nell' (girl's name).
'going back' says the letters should be written backwards.
'to get' means one lot of letters go next to another.
'nell' back-to-front is 'llen'.
'llen' put after 'a' is 'ALLEN'.
(Other definitions for allen that I've seen before include "Woody ..., US film maker" , "Woody ... is film director and comic actor" .)