I've been baffled too Smudgy and started looking into more profound wordplay, and thought I had No4 until I realised it was 8 letters, but this was my idea if it was 6 letters.
The train arrives this minute
The train - N American freight trains are called Box wagons or Box cars
(third boxcar, midnight train, destination Bangor Maine - Roger Miller)
=
Box
arrives - When the said train arrives, it gets in
in
this minute - take this as minute = tiny or small amount and not 60 seconds.
a small amount is a gram (Chambers = g)
g
Box-in-g BOXING
I'll run it up the flagpole and see if any of you salute it.