Regularly defends having clock outside running fast (6)
I believe the answer is:
speedy
'running fast' is the definition.
'regularly defends having clock outside' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'having clock' becomes 'spy' (I can't explain this - if you can you should give a lot more credence to this answer).
'outside' means one lot of letters goes inside another.
The alternate letters of 'defends' are 'eed'.
'eed' inserted into 'spy' is 'SPEEDY'.
(Other definitions for speedy that I've seen before include "Express" , "Fast-moving" , "Done quickly" , "Expeditious" , "'Fast, without delay (6)'" .)