Stomach jewellery rapidly losing value (8)
I believe the answer is:
tumbling
'rapidly losing value' is the definition.
(as in tumbling stock prices)
'stomach jewellery' is the wordplay.
'stomach' becomes 'tum' (synonyms).
'jewellery' becomes 'bling' (informal term for jewellery).
'tum'+'bling'='TUMBLING'
(Other definitions for tumbling that I've seen before include "Quickly declining" , "Falling down, as if collapsing" , "Rolling over and over, as a gymnast might" , "Falling headlong" , "Acrobatics" .)