Dine in The Athenaeum (3)
I believe the answer is:
eat
'dine' is the definition.
(eat a meal)
'in the athenaeum' is the wordplay.
'in' says the answer is hidden in the clue.
'EAT' can be found hidden inside 'the athenaeum'.
(Other definitions for eat that I've seen before include "Scoff" , "Munch" , "Take food" , "Consume (food)" , "Break bread" .)