Left in period of prosperity to flourish (5)
I believe the answer is:
bloom
'flourish' is the definition.
(thesaurus)
'left in period of prosperity' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'period of prosperity' becomes 'boom' (I've seen this before).
'l' placed into 'boom' is 'BLOOM'.
'to' acts as a link.
(Other definitions for bloom that I've seen before include "look good" , "Coat (lens); flower" , "Flush" , "Open" , "Flourish" .)