Marks smeared in glass (7)
I believe the answer is:
signals
'marks' is the definition.
'smeared in glass' is the wordplay.
'smeared' indicates an anagram.
'in'+'glass'='inglass'
'inglass' is an anagram of 'SIGNALS'.
(Other definitions for signals that I've seen before include "Actions that encode messages" , "(Traffic) lights" , "Flags, gestures" , "Indications or gestures as information or warning" , "Coded communications" .)