Assault with a nail, say (6)
I believe the answer is:
attack
'say' is the definition.
(I know that attack is a more specific form of the action criticise)
'assault with a nail' is the wordplay.
'assault with' becomes 'at' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a nail' becomes 'tack' (tack is a kind of nail).
'at'+'tack'='ATTACK'
Can you help me to learn more?
(Other definitions for attack that I've seen before include "Pitch into" , "Assault or criticise violently" , "Start a fight or criticise strongly" , "Offensive or inslaught" , "Fiercely criticise" .)