Without wheels, they run on external steel rims (7)
I believe the answer is:
sledges
'without wheels they run' is the definition.
The definition suggests an adverb but the answer is not.
'external steel rims' is the wordplay.
'external steel' becomes 'sl' (I can't explain this - if you can you should give a lot more credence to this answer).
'rims' becomes 'edges' (I've seen this before).
'sl'+'edges'='SLEDGES'
'on' is the link.
Can you help me to learn more?
(Other definitions for sledges that I've seen before include "More than one big hitter" , "Snow carts" , "Sleighs; hammers" , "insults" .)