Backchat? It's marginal (3)
I believe the answer is:
lip
I believe this is a double definition.
'backchat?' is the first definition.
(I've seen this in other clues)
'it's marginal' is the second definition.
(lip can mean an edge or margin)
(Other definitions for lip that I've seen before include "sauciness" , "In slang, impudent talk" , "Rim; impertinence" , "Brink" , "Fleshy fold of mouth tissue" .)