Informer in time making mistakes (6)
I believe the answer is:
errata
'mistakes' is the definition.
(erratum is a kind of mistake)
'informer in time' is the wordplay.
'informer' becomes 'rat' ('rat' can be a synonym of 'informer').
'in' means one lot of letters goes inside another.
'time' becomes 'era' (I've seen this before).
'rat' inserted within 'era' is 'ERRATA'.
'making' is the link.
(Other definitions for errata that I've seen before include "List of mistakes (in book)" , "Listed errors" , "Text mistakes" , "List of text corrections to be made in a publication" , "Mistakes (Latin)" .)