Speculator initially selling tin and gold (4)
'speculator' is the definition.
(type of stock market speculator)
'initially selling tin and gold' is the wordplay.
'initially' suggests taking the first letters.
The initial letters of 'selling tin and gold' is 'STAG'.
(Other definitions for stag that I've seen before include "Investor", "Hart", "speculator wanting quick profit", "Male-only", "This party is for men only".)