Useless at home, training pet (5)
I believe the answer is:
inept
'useless' is the definition.
(I've seen this before)
'home training pet' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'training' indicates an anagram (to train can mean to grow something into a shape).
'pet' with letters rearranged gives 'ept'.
'in'+'ept'='INEPT'
'at' acts as a link.
(Other definitions for inept that I've seen before include "Wanting" , "Unskilful, awkward" , "Ham-fisted" , "Silly - unskilled - unsuitable" , "Incompetent and ineffectual" .)