Brood over old-fashioned measure that’s seen at entrance (8)
I believe the answer is:
doorbell
'seen at entrance' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'brood over old-fashioned measure' is the wordplay.
'over' says the letters should be written backwards.
'old-fashioned measure' becomes 'ell' (historical unit of length. I am not sure about the 'old-fashioned' bit.).
'brood' in reverse letter order is 'doorb'.
'doorb'+'ell'='DOORBELL'
'that's' is the link.
Can you help me to learn more?
(Other definitions for doorbell that I've seen before include "Push button to ring" , "Entry ringer" , "Ring it for admittance" , "It signifies someone's arrival, perhaps" , "Bordello with signal fitted to entrance" .)