One testing tips from entrepreneurs invested in fool's gold (8)
I believe the answer is:
assessor
'one testing' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'tips from entrepreneurs invested in fool's gold' is the wordplay.
'tips from' means to remove the middle letters.
'invested in' indicates putting letters inside.
'fool's' becomes 'ass's' (both can mean a stupid person).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'entrepreneurs' with its middle removed is 'es'.
'es' going inside 'asss' is 'assess'.
'assess'+'or'='ASSESSOR'
Can you help me to learn more?
(Other definitions for assessor that I've seen before include "Judge of quality" , "One who rates for tax" , "Person valuing property" , "Examiner" , "Evaluator, estimator" .)