Caution had Simon dithering (8)
I believe the answer is:
admonish
'caution' is the definition.
(synonyms)
'had simon dithering' is the wordplay.
'dithering' indicates anagramming the letters.
'had'+'simon'='hadsimon'
'hadsimon' is an anagram of 'ADMONISH'.
(Other definitions for admonish that I've seen before include "Caution - reprimand" , "Mildly reprove" , "Forewarn" , "Tell off" , "Reprove, warn" .)