Tail is part of one and one is part of tail (6)
I believe the answer is:
rabbit
'tail' is the definition.
(thesaurus)
'tail is part of one and one is' is the wordplay.
I cannot quite understand how this works, but
'part' could be 'bit' (both mean a piece of something) and 'bit' is found in the answer.
'one' could be 'a' (a thing is one thing) and 'a' is present in the answer.
This explanation may well be incorrect...
'part of' acts as a link.
Can you help me to learn more?
(Other definitions for rabbit that I've seen before include "Roger, Peter or Brer" , "Meat" , "bun?" , "Cony" , "Burrow dweller" .)