Clue: We suspect it's a reputed 21-12er in 26 and 20s … (12)
Answer: GUARDIANISTA
Explanation: Definition: reputed 21-12er in 26 and 20s
 GUARDIAN (we) + (IT'S A) – a Guardianista is the stereotypical Guardian reader, so using "we" to clue the "Guardian" part of this is a bit weak