Unknown character close to stage, very much a fanatic (6)
I believe the answer is:
zealot
'a fanatic' is the definition.
(both can mean an extremist)
'unknown character close to stage very much' is the wordplay.
'unknown character' becomes 'z' (algebra).
'close to' indicates one should take the final letters (the close/ending of).
'very much' becomes 'a lot' (synonyms).
The last letter of 'stage' is 'e'.
'z'+'e'+'alot'='ZEALOT'
(Other definitions for zealot that I've seen before include "Fanatic, obsessed person" , "Fanatic for a cause" , "Person militant about a cause" , "Fanatical adherent" , "Fervent, perhaps militant, proponent" .)