Track down small animal, old, by where we live (3,2,5)
I believe the answer is:
run to earth
'track down' is the definition.
(I've seen this before)
'small animal old by where we live' is the wordplay.
'small animal' becomes 'runt' (runt of the litter).
'old' becomes 'o' (abbreviation).
'by' says to put letters next to each other.
'where we live' becomes 'earth' (I've seen this before).
'runt'+'o'+'earth'='RUN TO EARTH'
(Other definitions for run to earth that I've seen before include "Track down" , "Discover" , "Find after searching" , "Search out, find" , "what one must do with ground lead" .)