Monarch entertains a listener (3)
I believe the answer is:
ear
'listener' is the definition.
(body part which hears or listens)
'monarch entertains a' is the wordplay.
'monarch' becomes 'er' (abbreviation for Elizabeth Regina).
'entertains' indicates putting letters inside.
'er' going around 'a' is 'EAR'.
(Other definitions for ear that I've seen before include "cut" , "willingness to listen" , "Seed-bearing part of corn" , "Appreciation of sounds" , "Place for a plug" .)