Thanks for the feedback and advice everyone, it's reassuring to know it's happened to others too!
I disassemble my hihat stand and put the rod inside the tubing when I tear down the kit, so I'm not able to leave the clutch attached. I've been keeping it in my pedal case, but of course on this night I decided to bring a different pedal and the clutch didn't cross my mind at the time. The obvious solution is to put the clutch somewhere smarter (stickbag) and buy a spare to keep in my car as well. I got through the gig ok - it was '70s - '90s rock covers and there wasn't much need for any fancy footwork. I had it taped up initially to give me a "chck" sound, but it just sounded flat and dead. Eventually I removed all the tape and let them slosh - it wasn't too bad and I still got pretty decent stick definition over the noise floor of the guitar.
Larry, you said "There's no excuse not having spares....footpedal, bass drum heads, clutches, snare...everything essential. Spare car and house keys too." I play on average twice a week, and it's what's paying my way through uni - this might be a wake up call that I need to be more prepared in the future. I'll get a spare clutch, definitely. If I break a snare head at a gig I always figured I would call an early break and swap the head from my 14" floor tom, I figured I could gaff a torn bass drum head and I've never taken a spare pedal (or even a spare spring, nuts, footboard etc) to a gig. For playing locally in a pub covers band, taking a spare snare seems like overkill to me, but spare heads, clutches and a pedal I will definitely start doing, so thank you for the advice. I don't know what the heck I would do if I snapped a pedalboard at a gig..
For everyone else: how far do you go with taking spares to gigs? Does it depend on how important that gig is to you?