As De Niro might say, 'Take a seat' in an old chair (5)
I believe the answer is:
sedan
'an old chair' is the definition.
(as in a sedan chair)
'as de niro might say take a seat' is the wordplay.
I cannot really see how this works, but
'a' could be 'an' and 'an' is present in the answer.
The remaining letters 'sed' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'in' acts as a link.
(Other definitions for sedan that I've seen before include "Once a mobile chair for Danes" , "Four-door car" , "Chair to be carried by two" , "Saloon car (US)" , "Chair; car" .)