Speculate about reserve, where prices fall (4,6)
I believe the answer is:
bear market
'prices fall' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'speculate about reserve' is the wordplay.
'speculate' becomes 'bet' (I've seen this in other clues).
'about' is an insertion indicator.
'reserve' becomes 'earmark' ('earmark' can be a synonym of 'reserve'**).
'bet' enclosing 'earmark' is 'BEAR MARKET'.
'where' is the link.
(Other definitions for bear market that I've seen before include "Situation where there are more sellers than willing buyers" , "Situation with falling share prices" , "when stock prices fall" , "prices low in this" .)