The common upstart upset Saul! (5)
I believe the answer is:
usual
'the common' is the definition.
(I've seen this before)
'upstart upset saul' is the wordplay.
'upstart' becomes 'u' (start of the word 'up').
'upset' indicates anagramming the letters.
'saul' anagrammed gives 'sual'.
'u'+'sual'='USUAL'
(Other definitions for usual that I've seen before include "Normal, customary" , "Unexceptional" , "Accustomed" , "regarding customs?" , "Normal, habitual" .)