We must board the girl's chopper (5)
I believe the answer is:
hewer
'chopper' is the definition.
(I've seen this in another clue)
'we must board the girl's' is the wordplay.
'must board' indicates putting letters inside.
'the girl' becomes 'her' ('her' refers to a girl or woman).
'we' put inside 'her' is 'HEWER'.
(Other definitions for hewer that I've seen before include "Axeman" , "Nick" , "cutter" , "Person cutting (wood, say) with blows" .)