A man on the national terrorist watch list was smoked out by a sharp-eyed undercover cop yesterday, The Post has learned. Ashish Nayyar, an Indian national, was spotted puffing away on a cigarette by the plainclothes cop on the elevated No. 7 train platform at Queensboro Plaza around midnight Thursday, law-enforcement sources said. The officer issued him a ticket for smoking. When he did a warrant check on the smoker's name, he discovered Nayyar was on the terror watch list. Nayyar had a Patrolmen's Benevolent Association card on him from a relative who is on the force, police sources said.