Wench first to spurn sailor's arm (7)
I believe the answer is:
cutlass
'sailor's arm' is the definition.
(arm or weapon used by a sailor)
'wench first to spurn' is the wordplay.
'wench' becomes 'lass' (both can mean a girl or young woman).
'first' means one lot of letters go next to another.
'to spurn' becomes 'cut' (to cut off is to spurn).
'lass' after 'cut' is 'CUTLASS'.
(Other definitions for cutlass that I've seen before include "could hurt one, in the main" , "Pirate sword" , "Curved, one-edged sword" , "Sword formerly used by sailors" , "Old sailor's sword" .)