Start to burn as cheek punched by nasty thug (5,2)
I believe the answer is:
light up
'start to burn' is the definition.
(lighting up is a kind of starting to burn)
'cheek punched by nasty thug' is the wordplay.
'cheek' becomes 'lip' (both can mean insolence or rudeness).
'punched by' is an insertion indicator.
'nasty' indicates an anagram.
'thug' is an anagram of 'ghtu'.
'lip' going around 'ghtu' is 'LIGHT UP'.
'as' acts as a link.
(Other definitions for light up that I've seen before include "Start dragging in" , "Display joy" , "Start (a cigarette)" , "Become animated" , "To begin smoking" .)