Decide against boxing plan (4)
I believe the answer is:
idea
'plan' is the definition.
(I've seen this before)
'decide against boxing' is the wordplay.
'boxing' indicates a hidden word (the solution is contained or boxed up by the clue).
'IDEA' can be found hidden inside 'decide against'.
(Other definitions for idea that I've seen before include "Plan" , "Purpose" , "Thought, conception" , "Sudden inspiration?" , "Notion in the mind" .)