Mail say Labour's half-cut member is leading (6)
I believe the answer is:
armour
'mail say' is the definition.
(mail is a kind of armour)
'labour's half-cut member is leading' is the wordplay.
'half cut' means to take half the letters.
'member' becomes 'arm' (member can mean a limb).
'is leading' says to put letters next to each other.
'labour' with half the letters taken is 'our'.
'our' put after 'arm' is 'ARMOUR'.
(Other definitions for armour that I've seen before include "Suit of mail" , "Old warrior's suit" , "Metal or other protective plating for body" , "Protective covering for knight of old" , "Soldier's body protection" .)