Ideas that regularly ignored facts (4)
I believe the answer is:
data
'facts' is the definition.
(thesaurus)
'ideas that regularly ignored' is the wordplay.
'regularly ignored' means one should take alternating letters.
'ideas'+'that'='ideasthat'
The alternating letters of 'ideasthat' are 'DATA'.
(Other definitions for data that I've seen before include "Facts and figures as fed to computers (4)" , "Collection of bits of information" , "tables, perhaps" , "Information for/from computer" , "What's stored by computers" .)