He’ll have a stab at treating you (5)
I believe the answer is:
lance
'he'll have a stab at treating you' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'he'll have a' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is found in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for lance that I've seen before include "steel" , "Old cavalry weapon" , "Chap" , "Long weapon; prick" , "A horse soldier's spear" .)