Useless ornament to hang on ventilator (9)
I believe the answer is:
fandangle
'useless ornament' is the definition.
(I've seen this before)
'hang on ventilator' is the wordplay.
'hang' becomes 'dangle' (dangling is a kind of hanging).
'on' means one lot of letters go next to another.
'ventilator' becomes 'fan' (something which provides ventilation).
'dangle' put after 'fan' is 'FANDANGLE'.
'to' is the link.
(Other definitions for fandangle that I've seen before include "Nonsense" , "Useless ornament" .)