Fox in bother losing tail (3)
I believe the answer is:
tod
'fox' is the definition.
(historic term)
'bother losing tail' is the wordplay.
'bother' becomes 'to do' (I've seen this in another clue).
'losing tail' means to remove the last letter.
'todo' with its last letter taken off is 'TOD'.
'in' is the link.
(Other definitions for tod that I've seen before include "Sly person" , "Fox" , "Former English weight for wool - dot it" , "If you're on it, you're on your own" , "One's alone on this" .)