Gum stuck in tyre sinks in (5)
I believe the answer is:
resin
'gum' is the definition.
(I've seen this in another clue)
'stuck in tyre sinks in' is the wordplay.
'stuck in' indicates a hidden word.
'RESIN' is hidden within 'tyre sinks in'.
(Other definitions for resin that I've seen before include "Unprocessed pine secretion" , "Viscous fluid from tree sap" , "Elemi, for example" , "Sticky tree exudation" , "Rinse out this substance from trees" .)