Taunt rich setter, not well (4)
I believe the answer is:
Here is my best explanation:
'taunt' is the definition.
(jeering is a kind of taunting)
'rich setter not well' is the subsidiary indication.
'rich setter' becomes 'jeweller' (to set can mean to mount a stone in jewellery).
'not' suggests deleting specific letters.
'jeweller' with 'well' removed is 'jeer'.
(Other definitions for jeer that I've seen before include "Tease", "Taunt", "Mocking remark", "Deride - mock", "Mock - laugh at", "Mock publicly", "Mock rudely", "gibe", "Scoff, deride, mock" and "Barrack".)