When it got hot, this used to decrease (8)
I believe the answer is:
flatiron
'when it got hot this' is the definition.
The definition suggests a singular noun which matches the answer.
'to decrease' is the wordplay.
'to' becomes 'flat' (I can't explain this - if you can you should believe this answer much more).
'decrease' becomes 'iron' (de-crease: remove creases by ironing).
'flat'+'iron'='FLATIRON'
'used' is the link.
(Other definitions for flatiron that I've seen before include "Appliance once used for pressing clothes" , "household appliance" , "Antique household implement" , "Old-fashioned clothes presser" , "that's no longer a pressing requirement" .)