Theme: DUPES. The first word of each theme answer is a synonym for some kind of easily targeted victim.
16 A. *Sneaky blow : SUCKER PUNCH. A SUCKER is a gullible or easily deceived person. The related PUNCH is a cowardly hit, often from behind, delivered to an unsuspecting opponent.
55 A. *Peanuts : CHUMP CHANGE. A CHUMP is a foolish or easily deceived person. CHUMP CHANGE, aka peanuts, is an insignificant amount of money. But, insignificant to whom? YMMV.
10 D. *Place for brooding : PIGEON COOP. Nice word play. Brooding is the hatching of birds' eggs, and, in another context, thinking deeply about some troublesome matter. A PIGEON is someone easily swindled, or the victim of a con man. Cf Bernie Madoff.
26 D. *"Walkin' After Midnight" singer : PATSY CLINE. Ms Cline provides our theme song.
A PATSY is someone taken advantage of, either by cheating or taking the blame for something. Again, YMMV, but for my money this is the only theme answer that really closely corresponds to the unifier fill.
35 A. Easy mark ... and a hint to the starts of the answers to starred clues : FALL GUY. A scapegoat. I don't really see a FALL GUY as an easy mark for a swindle - though all the other theme entries do fit that description. So the whole thing seems a bit askew to me. Or am I being too persnickety?
Hi Gang - JzB here, taking the FALL on another Wednesday. My nits aside, this is a nicely constructed puzzle, with a pinwheel theme arrangement and the unifier in the middle. Let's take a spin and see what else we can uncover.