An inspiration to entertain (5)
I believe the answer is:
amuse
'to entertain' is the definition.
(amuse can mean to entertain or divert)
'an inspiration' is the wordplay.
'an' becomes 'a'.
'inspiration' becomes 'muse' (a muse is someone's inspiration).
'a'+'muse'='AMUSE'
(Other definitions for amuse that I've seen before include "Cause to laugh or smile" , "Cause mirth - distract" , "Keep occupied" , "Pleasantly occupy" , "Make (someone) laugh" .)