Run, legs buckling, carrying the twit (7)
I believe the answer is:
smuggle
'run' is the definition.
(I've seen this before)
'legs buckling carrying the twit' is the wordplay.
'buckling' indicates anagramming the letters.
'carrying' is an insertion indicator.
'the twit' becomes 'mug' (I can't explain this - if you can you should give a lot more credence to this answer).
'legs' is an anagram of 'sgle'.
'sgle' enclosing 'mug' is 'SMUGGLE'.
Can you help me to learn more?
(Other definitions for smuggle that I've seen before include "Adherent of religion founded by Guru Nanak" , "Import illegally" , "boot-leg" , "Import without paying customs duties" , "Bring in drugs" .)