Material on roof: that feature’s beginning to grate (9)
I believe the answer is:
thatching
'material on roof' is the definition.
(as in a thatched roof)
'that feature's beginning to grate' is the wordplay.
'feature' becomes 'chin' (chin is a kind of feature).
'beginning to' indicates taking the first letters.
The initial letter of 'grate' is 'g'.
'that'+'chin'+'g'='THATCHING'
(Other definitions for thatching that I've seen before include "roofing material ?" , "Roofing with straw or reeds" .)