Safety device half finished on purpose (4)
I believe the answer is:
fuse
'safety device' is the definition.
(electronic safety device)
'half finished on purpose' is the wordplay.
'half finished' becomes 'f' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'purpose' becomes 'use' (synonyms).
'f'+'use'='FUSE'
Can you help me to learn more?
(Other definitions for fuse that I've seen before include "Weld together through heat" , "Electrical fitting" , "Melt with heat" , "Tube or cord for igniting an explosive" , "Integrate" .)