Inflexible outfit I associated with duke (5)
I believe the answer is:
rigid
'inflexible' is the definition.
('rigid' can be a synonym of 'inflexible')
'outfit i associated with duke' is the wordplay.
'outfit' becomes 'rig' ('rig' can be a synonym of 'outfit'**).
'associated with' says to put letters next to each other.
'duke' becomes 'd'.
'rig'+'i'+'d'='RIGID'
(Other definitions for rigid that I've seen before include "strict" , "Unable to bend" , "Incapable of or resistant to bending" , "starched too much?" , "Stiff, unbending" .)