Upstart’s mean? Not the end of the world (7)
I believe the answer is:
parvenu
'upstart's' is the definition.
(synonyms)
'mean? not the end of the world' is the wordplay.
'mean?' becomes 'par' (I've seen this in other clues).
'not the end of' means to remove the last letter.
'the world' becomes 'venus' ('world' could refer to any planet).
'venus' with its last letter removed is 'venu'.
'par'+'venu'='PARVENU'
(Other definitions for parvenu that I've seen before include "One who climbs" , "One newly risen to wealth or power" , "Nouveau riche" , "Newly rich person" , "Arriviste" .)