Support trendy person with mental ability (5)
I believe the answer is:
brain
'person with mental ability' is the definition.
(brain can mean a brainy person)
'support trendy' is the wordplay.
'support' becomes 'bra' (a support for the breasts).
'trendy' becomes 'in' ('in' can mean fashionable).
'bra'+'in'='BRAIN'
(Other definitions for brain that I've seen before include "Intelligent person" , "It's in the skull" , "Clever type" , "Intellectual ability" , "Encephalon" .)