Skin is slashed first (5)
I believe the answer is:
cutis
'skin' is the definition.
(I've seen this before)
'is slashed first' is the wordplay.
'slashed' becomes 'cut' ('cut' can be a synonym of 'slashed').
'first' says to put letters next to each other.
'is' after 'cut' is 'CUTIS'.
(Other definitions for cutis that I've seen before include "Technical name for skin" , "Surface area" , "a bit skinny" , "Skin, dermis" , "skin, technically" .)