Delay in favour of carrying out nastier act (13)
I believe the answer is:
procrastinate
'delay' is the definition.
(I know that procrastinate is a kind of delay)
'in favour of carrying out nastier act' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'carrying' means one lot of letters go next to another (in a down clue, the bottom letters carry others).
'out' indicates an anagram.
'nastier'+'act'='nastieract'
'nastieract' with letters rearranged gives 'crastinate'.
'pro'+'crastinate'='PROCRASTINATE'
(Other definitions for procrastinate that I've seen before include "Stall" , "Delay taking action" , "Defer action" , "Postpone" , "Delay, waste time" .)