Breed with nurse with ultimately toned bum (4,3)
I believe the answer is:
rear end
'bum' is the definition.
(I know that bum can be written as rear end)
'breed with nurse with ultimately toned' is the wordplay.
'breed' becomes 'rear' (I've seen this in another clue).
'with' says to put letters next to each other.
'nurse' becomes 'en' (abbreviation for enrolled nurse).
'with' means one lot of letters go next to another.
'ultimately' indicates one should take the final letters.
The final letter of 'toned' is 'd'.
'rear'+'en'+'d'='REAR-END'
(Other definitions for rear end that I've seen before include "Crash into back of another vehicle" , "The buttocks (colloq.)" , "Run into another vehicle from behind" , "Backside (informal)" , "seat" .)