At home pet might be useless (5)
I believe the answer is:
inept
'useless' is the definition.
(I've seen this before)
'at home pet might be' is the wordplay.
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'might be' indicates an anagram.
'pet' with letters rearranged gives 'ept'.
'in'+'ept'='INEPT'
(Other definitions for inept that I've seen before include "Unskilful or unsuitable" , "Unskilful, awkward" , "Clumsy and incompetent" , "Maladroit" , "pointless" .)