light (a cigarette, cigar, pipe etc) up
Definition: to burn the end of a cigarette, cigar or pipe etc so that you can start smoking it.

(This phrasal verb has more than one meaning)
E.g.1: He put down the phone, sat back in his chair and lit up his cigar with a satisfied smile.
E.g.2: He was a horrible man who delighted in lighting up in ‘no smoking’ areas, just to annoy the people around him.

This phrasal verb can’t be separated.