A source of uranium mostly rich in gold (5)
I believe the answer is:
auric
'gold' is the definition.
(thesaurus)
'a source of uranium mostly rich' is the wordplay.
'source of' indicates taking the first letters.
'mostly' means to remove the last letter.
The first letter of 'uranium' is 'u'.
'rich' with its final letter taken away is 'ric'.
'a'+'u'+'ric'='AURIC'
'in' acts as a link.
(Other definitions for auric that I've seen before include "Of or containing gold" , "Golden" , "Pertaining to gold" , "Hugh -, Labour party leader" , "- Goldfinger (Bond)" .)