At home crazed pet is useless (5)
I believe the answer is:
inept
'useless' is the definition.
(I've seen this in another clue)
'at home crazed pet' is the wordplay.
'at home' becomes 'in' (similar in meaning).
'crazed' is an anagram indicator (I've seen this in other clues).
'pet' is an anagram of 'ept'.
'in'+'ept'='INEPT'
'is' acts as a link.
(Other definitions for inept that I've seen before include "Clumsy, out of place" , "Silly - unskilled - unsuitable" , "rubbish" , "Hopelessly incapable" , "Futile" .)