Attached safety device, on the day (5)
I believe the answer is:
fused
'attached' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?
'safety device on the day' is the wordplay.
'safety device' becomes 'fuse' (electronic safety device).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'the day' becomes 'd' (abbreviation for day).
'fuse'+'d'='FUSED'
Can you help me to learn more?
(Other definitions for fused that I've seen before include "Welded together by heat" , "Melted together" , "Coalesced" , "Combined" , "United" .)