Excellent compiler with good answer (4)
I believe the answer is:
mega
'excellent' is the definition.
(old-fashioned UK slang for 'very good')
'compiler with good answer' is the wordplay.
'compiler' becomes 'me' (the person who is writing these clues).
'with' says to put letters next to each other.
'good' becomes 'g' (abbreviation).
'answer' becomes 'a' (abbreviation eg in Q and A).
'me'+'g'+'a'='MEGA'
(Other definitions for mega that I've seen before include "Terrific" , "Game (anag) -- great" , "Great - denoting a million" , "ace" , "As brilliant" .)