Great answer, possibly brilliant in retrospect (4)
I believe the answer is:
mega
'great' is the definition.
(old-fashioned UK slang for 'very good')
'answer possibly brilliant in retrospect' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'possibly brilliant' becomes 'gem' (I've seen this in another clue. I am not sure about the 'possibly' bit.).
'in retrospect' is a reversal indicator (in retrospect means looking back).
'a'+'gem'='agem'
'agem' reversed gives 'MEGA'.
Can you help me to learn more?
(Other definitions for mega that I've seen before include "As brilliant" , "Big" , "Informally great or large" , "Huge (slang)" , "Enormous; very good" .)