Magnificent clothes knocked over excellent beer (7)
I believe the answer is:
regalia
'magnificent clothes' is the definition.
'regalia' can be an answer for 'clothes' (regalia is a kind of clothes). I am not sure about the 'magnificent' bit.
'knocked over excellent beer' is the wordplay.
'knocked over' is a reversal indicator.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'beer' becomes 'lager' (lager is a kind of beer).
'ai'+'lager'='ailager'
'ailager' reversed gives 'REGALIA'.
Can you help me to learn more?
(Other definitions for regalia that I've seen before include "Royal insignia" , "Ceremonial robes" , "Finery" , "Insignia of royalty or office" , "Royal items" .)