Some idiots ignore indicator (4)
I believe the answer is:
sign
'indicator' is the definition.
(thesaurus)
'some idiots ignore' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'SIGN' is hidden within 'idiots ignore'.
(Other definitions for sign that I've seen before include "Pointer - indicate agreement in writing" , "Autograph" , "Public display of a message" , "Indicator" , "Engage by written agreement" .)