Dared to clean round back of radiator (5)
I believe the answer is:
durst
'dared' is the definition.
(archaic past tense form for 'dare')
'clean round back of radiator' is the wordplay.
'clean' becomes 'dust' (dust can mean to remove dust from something).
'round' is an insertion indicator.
'back of' says to take the final letters.
The final letter of 'radiator' is 'r'.
'dust' going around 'r' is 'DURST'.
'to' acts as a link.
(Other definitions for durst that I've seen before include "defied once" , "Dared to venture" .)