Doctor Longden! Some synthetic fabric? (5)
I believe the answer is:
orlon
'synthetic fabric?' is the definition.
(I've seen this before)
'doctor longden some' is the wordplay.
'some' indicates the answer is hidden within the clue.
'ORLON' is hidden within 'doctor longden'.
(Other definitions for orlon that I've seen before include "Synthetic fabric" , "Synthetic acrylic fibre" , "Crease-resistant acrylic fabric" , "Synthetic fibre/fabric" .)