Clue: It prevents rust in gold finish to exotic metal in heavy tap (9)
Answer: INHIBITOR
Explanation: Definition: It prevents rust
A charade of 'in' plus HIBIT, an envelope ('in') of BI (bismuth, 'exotic metal') in HIT ('heavy tap') plus OR ('gold', with 'finish' indicating its place in the word). An inhibitor may prevent rust, but may hold back other chemical reactions.