Make mistake — Deirdre oddly ignored (3)
I believe the answer is:
err
'make mistake' is the definition.
(I've seen this before)
'deirdre oddly ignored' is the wordplay.
'oddly ignored' means one should take alternating letters.
The alternating letters of 'deirdre' are 'ERR'.
(Other definitions for err that I've seen before include "Stray from the straight and narrow" , "do something you shouldn't" , "Leave the straight and narrow" , "Sin" , "Transgress" .)