Blunder, as is possible with mule (3,4,4,2,2)
I believe the answer is:
put ones foot in it
'blunder' is the definition.
(I've seen this before)
'is possible with mule' is the wordplay.
I cannot quite see how this works, but
'possible' could be 'on' ('not on' can mean unacceptable or not possible) and 'on' is found within the answer.
This may be the basis of clue (or it may be nonsense).
'as' is the link.
(Other definitions for put ones foot in it that I've seen before include "Make a tactless blunder (3,4,4,2,2)" , "Commit a faux pas?" , "usual way to use a sock" , "as one might with one's trainer" , "Bungle" .)