Detain a fashionable bird (6)
I believe the answer is:
intern
'detain' is the definition.
(I know that intern is a more specific form of the action detain)
'a fashionable bird' is the wordplay.
'a fashionable' becomes 'in' (in can mean fashionable or trendy).
'bird' becomes 'tern' (tern is a kind of bird).
'in'+'tern'='INTERN'
(Other definitions for intern that I've seen before include "Deprive of freedom" , "Detain in custody" , "Supervised medical graduate" , "Incarcerate without trial - a junior hospital doctor?" , "Shut up" .)