I rant terribly when sat on by police? Definitely (3,7)
I believe the answer is:
for certain
'definitely' is the definition.
(I've seen this before)
'i rant terribly when sat on by police?' is the wordplay.
'terribly' indicates an anagram.
'when sat on by' says to put letters next to each other (in a down clue, some letters go above others).
'police?' becomes 'force' (the force can refer to the police force**).
'i'+'rant'='irant'
'irant' anagrammed gives 'rtain'.
'rtain' after 'force' is 'FOR CERTAIN'.
(Other definitions for for certain that I've seen before include "Assuredly" , "definitely" , "surely" , "without a doubt" .)