Part of heater is known to be a hazard (4)
I believe the answer is:
risk
'a hazard' is the definition.
(I know that hazard is a kind of risk)
'part of heater is known to' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'RISK' is hidden within 'heater is known to'.
'be' is the link.
(Other definitions for risk that I've seen before include "Possibility of bad outcome" , "Assessment of potential dangers -- any silk saris (anag)" , "Take a chance" , "Expose to danger" , "Gamble" .)