Upstart modifies silent tool (7)
I believe the answer is:
utensil
'tool' is the definition.
(I know that utensil is a type of tool)
'upstart modifies silent' is the wordplay.
'upstart' becomes 'u' (start of the word 'up').
'modifies' is an anagram indicator (I've seen 'modified' mean this).
'silent' anagrammed gives 'tensil'.
'u'+'tensil'='UTENSIL'
(Other definitions for utensil that I've seen before include "Useful tool" , "A kitchen pot or pan" , "Tool for practical use" , "Implement for practical use" , "knife, maybe" .)