They reflect, certainly after musical ending in failure (4-4)

'they reflect' is the definition.
(cat's-eyes are reflective road features)

'certainly after musical ending in failure' is the wordplay.
'certainly' becomes 'yes' (both can be used to answer in the affirmative).
'after' means one lot of letters go next to another.
'musical' becomes 'Cats' (Andrew Lloyd Webber musical).
'ending in' suggests the final letters.
The last letter of 'failure' is 'e'.
'cats'+'e'='catse'
'yes' put after 'catse' is 'CATS-EYES'.