Outrageously overrates true valuable find (8-5)
I believe the answer is:
treasure-trove
'valuable find' is the definition.
(a treasure trove is a valuable supply of something)
'outrageously overrates true' is the wordplay.
'outrageously' indicates anagramming the letters (I've seen this in other clues).
'overrates'+'true'='overratestrue'
'overratestrue' with letters rearranged gives 'TREASURE-TROVE'.
(Other definitions for treasure-trove that I've seen before include "Hidden store of valuables" , "Collection of delightful things" , "lots of good things" , "Valuables found hidden" .)