His opinion is sure to be of some value (8)
I believe the answer is:
assessor
'his opinion is sure to be of some' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'value' is the wordplay.
I cannot really see how this works, but
'value' could be 'assess' (synonyms) and 'assess' is present in the answer.
The remaining letters 'or' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for assessor that I've seen before include "Legal adviser" , "One who rates for tax" , "property valuer" , "Maker of valuations" , "One who estimates value etc for insurance company" .)