Mistakes rodent in age (6)
I believe the answer is:
errata
'mistakes' is the definition.
(an erratum is a mistake in a printed work)
'rodent in age' is the wordplay.
'rodent' becomes 'rat' (rat is a kind of rodent).
'in' is an insertion indicator.
'age' becomes 'era' (both can mean a long time).
'rat' placed within 'era' is 'ERRATA'.
(Other definitions for errata that I've seen before include "Publishing mistakes" , "Mistakes in printed work" , "Mistakes in printed proofs" , "List of printed mistakes" , "Noted mistakes" .)