Beard not properly shaven: could it be the last straw? (7)
I believe the answer is:
stubble
I believe this clue is a double definition.
'beard not properly shaven' is the first definition.
(small amount of facial hair)
'could it be the last straw?' is the second definition.
(term for straw left after grain has been harvested)
(Other definitions for stubble that I've seen before include "Short stiff hairs on a man's face before a shave" , "Unshaven growth on the face" , "Post-harvest stalks" , "Cut stalks left in field" , "Short whiskers" .)