Alert, he replaced material (7)
I believe the answer is:
leather
'material' is the definition.
(I've seen this before)
'alert he replaced' is the wordplay.
'replaced' indicates anagramming the letters.
'alert'+'he'='alerthe'
'alerthe' is an anagram of 'LEATHER'.
(Other definitions for leather that I've seen before include "durable material" , "Treated hide" , "Whip" , "Fabric" , "kid, say" .)