Folk hero is to steal, wearing mask (5,4)
I believe the answer is:
robin hood
'folk hero' is the definition.
'to steal wearing mask' is the wordplay.
'to steal' becomes 'rob' (robbing is a kind of stealing).
'wearing' becomes 'in' (wear in (and 'in' is fashionable)).
'mask' becomes 'hood'.
'rob'+'in'+'hood'='ROBIN HOOD'
'is' acts as a link.
(Other definitions for robin hood that I've seen before include "Folk hero of Sherwood Forest - robbed the rich and gave to the poor" , "one favouring wealth redistribution" , "English folk hero" , "Chief of the Merry Men" , "Famous outlaw" .)