Put money in waistcoat (6)
I believe the answer is:
invest
'put money' is the definition.
(I've seen this before)
'in waistcoat' is the wordplay.
'waistcoat' becomes 'vest' (synonyms).
'in'+'vest'='INVEST'
(Other definitions for invest that I've seen before include "Put money into shares, property, etc" , "Eg, buy stocks and shares" , "Play the market" , "Put money into business for profit" , "Put money into a company" .)