Motorway strangely clear — an amazing occurrence (7)
I believe the answer is:
miracle
'amazing occurrence' is the definition.
(I've seen this before)
'motorway strangely clear an' is the wordplay.
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'strangely' is an anagram indicator.
'an' becomes 'i' (Roman numeral for one).
'clear'+'i'='cleari'
'cleari' anagrammed gives 'iracle'.
'm'+'iracle'='MIRACLE'
(Other definitions for miracle that I've seen before include "It's said to happen by the divine intervention" , "Inexplicable wonder" , "Welcome event that's not explained by natural law" , "Event that appears to break the law of nature" , "Supernatural happening" .)