Large blade with phoney coating's a cracker (12)
I believe the answer is:
sledgehammer
'cracker' is the definition.
(I know that sledgehammer is a type of cracker)
'large blade with phoney coating's a' is the wordplay.
I cannot quite understand how this works, but
'large' could be 'l' (eg in clothes sizes) and 'l' is found in the answer.
'blade' could be 'edge' (as in a sharpened edge) and 'edge' is found within the answer.
'phoney' could be 'sham' (I've seen this before) and 'sham' is found in the leftover letters.
The remaining letters 'mer' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for sledgehammer that I've seen before include "Dealer of heavy blow" , "Unsubtle" , "One violently smashing" , "One may be used to break rocks" , "Tool" .)