Sign me on, mistakenly (4)
I believe the answer is:
omen
'sign' is the definition.
(I've seen this before)
'me on mistakenly' is the wordplay.
'mistakenly' is an anagram indicator.
'me'+'on'='meon'
'meon' with letters rearranged gives 'OMEN'.
(Other definitions for omen that I've seen before include "Warning token" , "Sign of something about to happen" , "Portent of things to come" , "Prophetic significance" , "writing on the wall" .)