A note in case of late error (5)
I believe the answer is:
lapse
'error' is the definition.
(lapse is a kind of error)
'a note in case of late' is the wordplay.
'note' becomes 'ps' (PS is an example**).
'in' means one lot of letters goes inside another.
'case of' says to hollow out the word (remove centre letters) (outsides of).
'late' with its middle taken out is 'le'.
'a'+'ps'='aps'
'aps' going inside 'le' is 'LAPSE'.
(Other definitions for lapse that I've seen before include "Pleas about omission" , "Fail in duty" , "Slip back" , "Slip, error" , "Decline into an inferior state" .)