Sack women on non-professional new seat (3,5)
I believe the answer is:
lay waste
'sack' is the definition.
(to sack can mean to destroy a town or city)
'women on non-professional new seat' is the wordplay.
'women' becomes 'w'.
'on' means one lot of letters goes inside another.
'non-professional' becomes 'lay' (I've seen this before).
'new' is an anagram indicator.
'seat' with letters rearranged gives 'aste'.
'lay'+'aste'='layaste'
'w' inserted within 'layaste' is 'LAY WASTE'.
(Other definitions for lay waste that I've seen before include "Destroy and ruin an area" , "Devastate" , "Destroy or ruin, demolish" , "Raze, destroy buildings, crops etc" , "Make desolate" .)