In the vaguest sense, one being entertained (5)
I believe the answer is:
guest
'one being entertained' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'in the vaguest sense' is the wordplay.
'in' indicates the answer is hidden within the clue.
'GUEST' is hidden within 'vaguest sense'.
Can you help me to learn more?
(Other definitions for guest that I've seen before include "Caller" , "Visitant" , "Visitor to whom hospitality is extended" , "Recipient of hospitality" , "Person entertained by host" .)