Old hat banned by doctor penning poem (8)
I believe the answer is:
outmoded
'old hat' is the definition.
(thesaurus)
'banned by doctor penning poem' is the wordplay.
'banned' becomes 'out'.
'by doctor' becomes 'md' ('MD' can be a synonym of 'doctor'. I am not sure about the 'by' bit.**).
'penning' means one lot of letters goes inside another.
'poem' becomes 'ode' (ode is a kind of poem).
'md' going around 'ode' is 'moded'.
'out'+'moded'='OUTMODED'
(Other definitions for outmoded that I've seen before include "No longer fashionable" , "Dated" , "No longer in fashion" , "Old-fashioned" .)