where the sheet metal chimney pipe exits from the plenum of my (oil,
hot air) furnace, there is almost an inch gap all around it. this is on
the suction side of the fan, just below the filter, which seems to be
not getting dirty at nearly the rate I'm used to in previous homes. I'm
wondering if, in succession:
1) is this gap unusually large?
2) is there enough basement air getting in through the gap to short
circuit the filter?
3) is there something I can plug the gap with that will not catch on
fire or emit nasty fibres or such into the airstream?
Thanks in advance, all.

Something's a little unclear here, your chimney pipe should NOT be at
all associated with the return plenum (think about it: why would you
vent the harmful burner exhaust into the return side of the fan?).

I'm going to guess you mean that the return duct has a gap. If that's
the case, #1 yes it's unusually large, #2 yes there probably is a lot
of basement air bypassing the filter, and yes you should seal it up.
If it's not too large (you mention 1" wide), you might be able to tape
it up with foil tape. If it's too large for tape alone, you can
probably use some aluminum flashing to cover the gap. Either tape the
edges down or pop-rivet them or use sheet-metal screws (THEN tape the
edges to seal it up).

If sealing this up means that there won't be enough return air being
pulled into the fan, you can maybe add a return grille in the plenum
above the filter so that the air does pass through the filter.

Take my advice with a grain of salt, I'm just a home owner not a
professional.