Blow is hard, punching face (6)
I believe the answer is:
mishap
'blow' is the definition.
(I've seen this before)
'is hard punching face' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'punching' is an insertion indicator (some letters punch a hole in others).
'face' becomes 'map' (I've seen this before).
'is'+'h'='ish'
'ish' put into 'map' is 'MISHAP'.
(Other definitions for mishap that I've seen before include "An accident, piece of bad luck" , "Unfortunate minor accident" , "Stroke of bad luck" , "Unlucky development" , "Blow" .)