Forecaster, we hear, making financial gain (6)
I believe the answer is:
profit
'financial gain' is the definition.
(profit is a kind of financial gain)
'forecaster we hear' is the wordplay.
'forecaster' becomes 'prophet' (I've seen this before).
'we hear' indicates a 'sounds like' (homophone) clue.
'prophet' sounds like 'PROFIT'.
'making' is the link.
(Other definitions for profit that I've seen before include "bonus" , "Use" , "returns" , "Gain from employment of capital" , "Surplus earnings" .)