Busy with leader of party out for change (2,2)
I believe the answer is:
up to
'busy with' is the definition.
(I've seen this before)
'leader of party out for change' is the wordplay.
'leader of' says to take the initial letters.
'for change' indicates an anagram (I've seen this in other clues).
The first letter of 'party' is 'p'.
'p'+'out'='pout'
'pout' with letters rearranged gives 'UP TO'.
(Other definitions for up to that I've seen before include "As far as a certain point" , "As many as, though no more" , "Incumbent upon" , "As far as or as many as" , "Busy with" .)