Yearly warning unable to be ignored on regular basis (6)
I believe the answer is:
annual
'yearly' is the definition.
(synonyms)
'warning unable to be ignored on regular basis' is the wordplay.
'to be ignored on regular basis' indicates alternate letters.
'warning'+'unable'='warningunable'
The alternate letters of 'warningunable' are 'ANNUAL'.
(Other definitions for annual that I've seen before include "plant > book" , "Lasting for a year" , "Happening once each year" , "Occuring once a year" , "Plant; yearbook" .)