Wheel crossing line causes obstruction (4)
I believe the answer is:
clog
'obstruction' is the definition.
(I know that clog is a type of encumbrance)
'wheel crossing line' is the wordplay.
'wheel' becomes 'cog' (wheel with teeth in a machine).
'crossing' is an insertion indicator.
'line' becomes 'l' (used when specifying particular lines from a poem).
'cog' going around 'l' is 'CLOG'.
'causes' acts as a link.
(Other definitions for clog that I've seen before include "Block with wooden-soled shoe?" , "Block; sabot" , "Bung up" , "Jam" , "Wooden shoe; choke up" .)