Top grade, as it happens, existing (5)
I believe the answer is:
alive
'existing' is the definition.
(similar in meaning)
'top grade as it happens' is the wordplay.
'top grade' becomes 'A' (top academic grade).
'as it happens' becomes 'live' (as in live television).
'a'+'live'='ALIVE'
(Other definitions for alive that I've seen before include "Breathing, existing" , "Aware" , "Alert, active" , "Functioning" , "answer, as it happens" .)