Young upstart initially invested in Marmite spread (8)
I believe the answer is:
immature
'young' is the definition.
('immature' can be a synonym of 'young')
'upstart initially invested in marmite spread' is the wordplay.
'initially' suggests taking the first letters.
'invested in' is an insertion indicator.
'spread' indicates anagramming the letters.
The first letter of 'upstart' is 'u'.
'marmite' with letters rearranged gives 'immatre'.
'u' put into 'immatre' is 'IMMATURE'.
(Other definitions for immature that I've seen before include "Young" , "Undeveloped - unripe" , "behaviour of child" , "Not yet adult or fully developed" , "Not fully ripe or developed" .)