Runner having to start waddling up a mountain! (7)
I believe the answer is:
skiddaw
'to start waddling up a mountain' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'runner' is the wordplay.
I cannot quite see how this works, but
'runner' could be 'skid' (I've seen this before) and 'skid' is located in the answer.
The remaining letters 'daw' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'having' acts as a link.
Can you help me to learn more?
(Other definitions for skiddaw that I've seen before include "Cumbrian mountain" , "Mountain of Cumbria" , "Lake District mountain" , "fell" .)