Mistake by coach with small weapon (11)
I believe the answer is:
blunderbuss
'weapon' is the definition.
(blunderbuss is a kind of weapon)
'mistake by coach with small' is the wordplay.
'mistake by' becomes 'blunder' (blunder is a kind of mistake. I am not sure about the 'by' bit.).
'coach' becomes 'bus' (synonyms).
'with' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'blunder'+'bus'+'s'='BLUNDERBUSS'
Can you help me to learn more?
(Other definitions for blunderbuss that I've seen before include "one was fired" , "Short large-bore musket with flared muzzle" , "Early form of gun" , "Obsolete musket" , "Weapon" .)