Rearing up, tame animals cross threshold (8)
I believe the answer is:
doorstep
'threshold' is the definition.
(I know that threshold can be written as doorstep)
'rearing up tame animals cross' is the wordplay.
'rearing up' shows that the letters should be reversed in order (as in a horse rearing up).
'tame animals' becomes 'pets' (domesticated or tamed animals).
'cross' becomes 'rood' (term for a crucifix).
'pets'+'rood'='petsrood'
'petsrood' written backwards gives 'DOORSTEP'.
(Other definitions for doorstep that I've seen before include "Threshold stair" , "Canvass locally" , "Go from house to house" , "Try some canvassing" , "go canvassing" .)