Boxers? Nancies' pants (7)
I believe the answer is:
Here is my best explanation:
'boxers?' is the definition.
(boxer is a kind of canine)
'nancies' pants' is the subsidiary indication.
'pants' indicates anagramming the letters (UK informal term for \'nonsense\' or \'rubbish\').
'nancies' is an anagram of 'canines'.
(Other definitions for canines that I've seen before include "Dogs -- teeth" and "Teeth".)