Slowing down where article blocks crumbling dirt road (10)
I believe the answer is:
ritardando
'slowing down' is the definition.
(I've seen this before)
'article blocks crumbling dirt road' is the wordplay.
'article' becomes 'an' (indefinite article).
'blocks' indicates putting letters inside (upper walls off glass inside formal).
'crumbling' indicates anagramming the letters.
'dirt'+'road'='dirtroad'
'dirtroad' anagrammed gives 'ritarddo'.
'an' going within 'ritarddo' is 'RITARDANDO'.
'where' acts as a link.
(Another definition for ritardando that I've seen is " slowing down".)