Tyres openly moulded in synthetic material (11)
I believe the answer is:
polystyrene
'synthetic material' is the definition.
(I've seen this before)
'tyres openly moulded' is the wordplay.
'moulded' indicates anagramming the letters.
'tyres'+'openly'='tyresopenly'
'tyresopenly' is an anagram of 'POLYSTYRENE'.
'in' acts as a link.
(Other definitions for polystyrene that I've seen before include "Hard plastic" , "Lightweight synthetic material" , "Light synthetic resin" , "Synthetic resin for packing, insulation" , "Moulded synthetic material" .)