Warning: sharp cut will lead to rueful expression (4,3)
I believe the answer is:
keep out
'warning' is the definition.
(eg a warning to trespassers)
'sharp cut will lead to rueful expression' is the wordplay.
'sharp' becomes 'keen' (synonyms).
'cut' means to remove the last letter.
'will lead' means one lot of letters go next to another.
'to rueful expression' becomes 'pout' (sulking facial expression).
'keen' with its final letter taken away is 'kee'.
'kee'+'pout'='KEEP OUT'
(Other definitions for keep out that I've seen before include "warning note to trespassers?" , "No entry!" , "Exclude" , "notice to trespassers?" , "Stay clear" .)