Idiot, alien initially stumble upon valuables (6)
I believe the answer is:
assets
'valuables' is the definition.
(I've seen this in another clue)
'idiot alien initially stumble' is the wordplay.
'idiot' becomes 'ass' (I've seen this in other clues).
'alien' becomes 'et' (extra terrestrial).
'initially' suggests taking the first letters.
The first letter of 'stumble' is 's'.
'ass'+'et'+'s'='ASSETS'
'upon' acts as a link.
(Other definitions for assets that I've seen before include "capital" , "Personal wealth" , "One's total goods" , "Useful or valuable possessions or qualities" , "Stance discouraging particular action" .)