A thought entertained (6)
I believe the answer is:
amused
'entertained' is the definition.
(amusing is a kind of entertaining)
'a thought' is the wordplay.
'thought' becomes 'mused' (musing is a kind of thinking).
'a'+'mused'='AMUSED'
(Other definitions for amused that I've seen before include "Made (someone) laugh" , "Queen Victoria wasn't!" , "Mildly entertained by Medusa" , "Pleasantly occupied" , "Medusa (anag)" .)