My pet might be vain (5)
I believe the answer is:
empty
'vain' is the definition.
(I've seen this before)
'my pet might be' is the wordplay.
'might be' indicates an anagram.
'my'+'pet'='mypet'
'mypet' with letters rearranged gives 'EMPTY'.
(Other definitions for empty that I've seen before include "Void; vacant" , "vain" , "'. . . . . vessels make the most sound'" , "Arabic quarter" , "Drained glass" .)