A little thing fades, going belly-up, then sadly gone (8)
I believe the answer is:
smidgeon
'a little thing' is the definition.
(I know that smidgeon is little)
'fades going belly-up then sadly gone' is the wordplay.
'fades' becomes 'dims' (I've seen this in another clue).
'going belly-up' is a reversal indicator (in a down clue, letters go up).
'then' says to put letters next to each other.
'sadly' indicates an anagram.
'dims' back-to-front is 'smid'.
'gone' with letters rearranged gives 'geon'.
'smid'+'geon'='SMIDGEON'
(Other definitions for smidgeon that I've seen before include "Tiny amount" , "Very small part" , "Small portion" , "Small amount" , "A little" .)