Definitely compel downfall - about time! (3,7)
I believe the answer is:
for certain
'definitely' is the definition.
(I've seen this before)
'compel downfall about time' is the wordplay.
'compel' becomes 'force' (forcing is a kind of compelling).
'downfall' becomes 'rain' (rain is a kind of downfall**).
'about' is an insertion indicator.
'time' becomes 't'.
'rain' going around 't' is 'rtain'.
'force'+'rtain'='FOR CERTAIN'
(Other definitions for for certain that I've seen before include "without a doubt" , "surely" , "definitely" , "Assuredly" .)