Smears mark broken signal (7)
I believe the answer is:
maligns
'smears' is the definition.
(maligning is a kind of smearing)
'mark broken signal' is the wordplay.
'mark' becomes 'm'.
'broken' indicates anagramming the letters.
'signal' is an anagram of 'aligns'.
'm'+'aligns'='MALIGNS'
(Other definitions for maligns that I've seen before include "Badmouths" , "Defames" , "Says unpleasant and untrue things about" .)