Dog having white heifer as companion? That's an unusual thing (5)
I believe the answer is:
curio
'unusual thing' is the definition.
'dog having white heifer' is the wordplay.
'dog' becomes 'cur' (cur is a kind of dog).
'having' means one lot of letters go next to another.
'white heifer' becomes 'io' (girl changed into a heifer in Greek myth).
'cur'+'io'='CURIO'
'as companion? that's an' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for curio that I've seen before include "Unusual, intriguing object" , "Unusual object, oddity" , "something odd" , "Rare or unusual thing" , "Bygone" .)