Top-class gunmen conserve energy but waste material (4)
I believe the answer is:
urea
'waste material' is the definition.
(I've seen this before)
'top-class gunmen conserve' is the wordplay.
'top-class' becomes 'u' (as in U and non-U speech).
'gunmen conserve' becomes 'rea' (I can't justify this - if you can you should believe this answer much more).
'u'+'rea'='UREA'
'energy but' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for urea that I've seen before include "Human waste product" , "Compound found in urine, synthesised as a fertiliser" , "It's produced by piddling" , "Waste product of mammals" , "Waste material" .)