Forget I'm to about turn (4)
I believe the answer is:
omit
'forget' is the definition.
(to omit to do something is to forget to do it)
'i'm to about turn' is the wordplay.
'about' means one lot of letters goes inside another.
'turn' shows that the letters should be reversed in order.
'im' inserted within 'to' is 'timo'.
'timo' in reverse letter order is 'OMIT'.
(Other definitions for omit that I've seen before include "Leave out or forget" , "Take out" , "Exclude; forget to include" , "Miss out" , "Leave out or undone" .)