Veteran is fooled from time to time (3)
I believe the answer is:
old
'veteran' is the definition.
(thesaurus)
'fooled from time to time' is the wordplay.
'from time to time' means one should take alternating letters (letters removed on a regular basis).
The alternating letters of 'fooled' are 'OLD'.
'is' is the link.
(Other definitions for old that I've seen before include "Ancient" , "Getting on" , "Obsolete" , "Former, previous" , "Senescent" .)