On the up, some prosper eventually -- or not (5)
I believe the answer is:
never
'not' is the definition.
(thesaurus)
'on the up some prosper eventually' is the wordplay.
'on the up' is a reversal indicator (in a down clue, letters go up).
'some' indicates a hidden word (some of the parts of the word are used).
'reven' can be found hidden inside 'prosper eventually'.
'reven' reversed gives 'NEVER'.
'or' is the link.
(Other definitions for never that I've seen before include "I refuse" , "On no occasion" , "Emphatic negative" , "At no time in the past or future" , "' - - Say - - Again': Bond film" .)