Rating coat as outdoor covering (6)
I believe the answer is:
tarmac
'outdoor covering' is the definition.
'rating coat' is the wordplay.
'rating' becomes 'tar'.
'coat' becomes 'mac' (mac is a kind of coat**).
'tar'+'mac'='TARMAC'
'as' is the link.
(Other definitions for tarmac that I've seen before include "Road paving material" , "black one" , "Road-surfacing material" , "Airport runway surface" , "Road-surfacing substance" .)