One who set the pace is back to sum up (5)
I believe the answer is:
recap
'sum up' is the definition.
(recapping is a kind of summing up)
'one who set the pace is back' is the wordplay.
'one who set' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'is back' says the letters should be written backwards.
'pace' written backwards gives 'ecap'.
'r'+'ecap'='RECAP'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for recap that I've seen before include "Restate" , "Sum up (abbrev.)" , "Caper (anag)" , "Go over again with backward pacer" , "Repeat of main points" .)