Very large group of nations - very good to get involved (4)
I believe the answer is:
epic
'very large' is the definition.
'group of nations very good to get involved' is the wordplay.
'group of nations' becomes 'ec' (abbreviation for European Commission).
'very good' becomes 'pi' (I've seen this in other clues).
'to get involved' indicates putting letters inside.
'ec' going around 'pi' is 'EPIC'.
(Other definitions for epic that I've seen before include "Long, heroic story" , "major work" , "Long poem - awesome!" , "Long poem set in nice picture" , "Heroic, huge" .)