The Pilgrims were beaten 2-0 by Gloucester City at the TigerTurf Stadium – courtesy of McClure's 12th minute penalty and a clincher four minutes from time.
United were improved after the break, with substitutes Micah Obiero and Jake Wright lively in attack, but were unable to find a way through the City rearguard.
Sam Long made his United debut and made a brilliant save to deny Daniel Leadbitter within five minutes. Kieran Phillips subsequently missed a glorious chance to score at the back post three minutes later.
United's luck ran out when Scott Pollock fouled Sion Spence and McCLURE sent the resulting spot-kick straight down the middle.
Jordon Crawford sent a free header wide after Alex Brown's cross had been hooked on by Scott Pollock.
Ben Pollock's header then bounced off the goal frame following Joe Leesley's ball in from the right-hand side.
United exerted some second half pressure, without seriously threatening Brandon Hall in the City goal. Wright whistled an effort across the face of goal and wide, when he arguably should have done better.
McCLURE wrapped up the points with a driven effort from 20 yards with four minutes to play.
Gloucester (3-4-1-2): Brandon Hall; Daniel Leadbitter, Ben Nugent (Jamie Egan 50), Kennedy Digie; Jack James, Kevin Dawson, Ben Morgan, Owen Evans; Sion Spence (Lucas Tomlinson 90); Matt McClure, Kieran Phillips (Tre Mitford 68). Subs (not used): Johnny Johnston, Liam Armstrong (gk).
Boston (3-4-2-1): Sam Long; Zak Mills, Luke Shiels, Ben Pollock; Ellis Myles, Brad Abbott, Joe Leesley, Alex Brown; Jordon Crawford (Micah Obiero 37), Scott Pollock; Jaanai Gordon (Jake Wright 58). Subs (not used): Lebrun Mbeka, Keenan Ferguson, Will Atkinson.
Referee: Matthew Scholes.
Attendance: 798.
PIC: Neil Phelps.