Heavy weight going north (3)
I believe the answer is:
ton
'heavy weight' is the definition.
(a ton weighs a lot)
'going north' is the wordplay.
I cannot quite see how this works, but
'going' could be 'on' (both can mean functioning or activated) and 'on' is located in the answer.
A single letter 't' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for ton that I've seen before include "A slang 100 mph" , "A slangy 100mph - heavy going" , "Two thousand - two hundred and forty pounds" , "100 mph, runs, etc" , "Twenty cwt" .)