Pop up, running clean to the top (8)
I believe the answer is:
pinnacle
'the top' is the definition.
('pinnacle' can be a synonym of 'top')
'pop up running clean' is the wordplay.
'pop' becomes 'nip' ('nip out' and 'pop out' can both mean to go out).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'running' is an anagram indicator.
'nip' reversed gives 'pin'.
'clean' is an anagram of 'nacle'.
'pin'+'nacle'='PINNACLE'
'to' acts as a link.
(Other definitions for pinnacle that I've seen before include "Topmost point" , "Towering peak" , "Highest point (of fame or building)" , "Acme" , "The highest point, peak" .)