Wise to avoid exterior ruin (3)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

[s] MAR [t] (wise), avoding the outer letters (exterior). Not sure about the definition: to me, “mar” = to damage or disfigure something, suggesting rather less than “ruin” = to destroy it or make it unusable. The difference between scratching your car’s paintwork and writing it off in an accident.