His or her model beneficiary (5)
I believe the answer is:
their
'his or her' is the definition.
(I've seen this before)
'model beneficiary' is the wordplay.
'model' becomes 't' (Ford Model T car).
'beneficiary' becomes 'heir'.
't'+'heir'='THEIR'
(Other definitions for their that I've seen before include "Belonging to him (perhaps wrongly)" , "belonging to an individual" , "Relating to them" , "Possessive pronoun" , "Some would say his (or her)" .)