Strip of metal or wood to chuck outside (8)
I believe the answer is:
flashing
'strip of metal' is the definition.
(strip of metal on a roof)
'wood to chuck outside' is the wordplay.
'wood' becomes 'ash' (wood from an ash tree).
'to chuck' becomes 'fling' (I've seen this before).
'outside' indicates putting letters inside.
'ash' inserted within 'fling' is 'FLASHING'.
'or' acts as a link.
(Other definitions for flashing that I've seen before include "Shaped metal attached to a roof for strength and weatherproofing" , "Weather-proofing strip - like a strobe" , "Waterproof strip" .)