Not eating a little butter? Well remembered (3,3)
I believe the answer is:
off pat
'well remembered' is the definition.
(to know something off-pat is to memorise it)
'not eating a little butter?' is the wordplay.
'not eating' becomes 'off' (eg someone might be 'off' dairy).
'a little butter?' becomes 'pat' ('pat' can mean a lump of butter).
'off'+'pat'='OFF PAT'
(Other definitions for off pat that I've seen before include "Recalled perfectly" , "Fluent" , "Faithfully retained" , "exactly recalled" , "Perfectly memorised" .)