Entertainer, mature, one detained (7)
I believe the answer is:
hostage
'one detained' is the definition.
(a hostage is detained against their will)
'entertainer mature' is the wordplay.
'entertainer' becomes 'host' (person hosting or entertaining guests).
'mature' becomes 'age' (synonyms).
'host'+'age'='HOSTAGE'
(Other definitions for hostage that I've seen before include "One held for money" , "Prisoner - pawn" , "She-goat (anag)" , "Kidnap victim" , "Person held by kidnapper for ransom" .)