Family dog returns ball endlessly (3)
I believe the answer is:
lab
'family dog' is the definition.
(I've seen this in another clue)
'returns ball endlessly' is the wordplay.
'returns' says the letters should be written backwards.
'endlessly' means to remove the last letter.
'ball' with its last letter removed is 'bal'.
'bal' written backwards gives 'LAB'.
(Other definitions for lab that I've seen before include "Science room" , "Experimental facility" , "Scientists' workplace" , "Science workshop" , "retriever" .)