Mischievous devil Knut upset Prime Minister (3)
I believe the answer is:
imp
'mischievous devil' is the definition.
(I've seen this before)
'knut upset prime minister' is the wordplay.
'knut' becomes 'I' (where the crossword setter is 'Knut').
'upset' shows that the letters should be reversed in order.
'prime minister' becomes 'PM' (abbreviation).
'pm' written backwards gives 'mp'.
'i'+'mp'='IMP'
(Other definitions for imp that I've seen before include "Little troublemaker" , "Small mischievous sprite" , "Small devil or sprite" , "Small mischievous spirit" , "Young troublemaker" .)