8 down
Donates,
as
replacing
4,
6
and
15
(5)
Donates, as replacing 4, 6 and 15 (5)
hints
0
checks
0
hints
The definition is at the end of the clue.
How the clue works
The definition is 6 and 15 — those numbers point you to the answers at 6 down (OXYGEN) and 15 across (NEON), both of which are gases, so that's what you're after. For the wordplay, take Donates, which gives you GIVES, then as replacing 4 means you swap out IV (the Roman numeral for 4) and drop in AS instead, leaving you with G, then AS, then ES.