Welsh parliamentarian has way of working material for attack (4)
I believe the answer is:
ammo
'material for attack' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'welsh parliamentarian has way of working' is the wordplay.
'welsh parliamentarian has' becomes 'am' (I can't justify this - if you can you should give a lot more credence to this answer).
'way of working' becomes 'mo' (modus operandi).
'am'+'mo'='AMMO'
Can you help me to learn more?
(Other definitions for ammo that I've seen before include "What's fired from a gun (abbr)" , "arms" , "Bullets (abbrev.)" , "Bullets etc.(informal)" , "Shot" .)