Stupid at home, his new shrink (8)
I believe the answer is:
diminish
'shrink' is the definition.
(I know that shrink is a more specific form of diminish)
'stupid at home his new' is the wordplay.
'stupid' becomes 'dim' (similar in meaning).
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'new' indicates anagramming the letters.
'his' is an anagram of 'ish'.
'dim'+'in'+'ish'='DIMINISH'
(Other definitions for diminish that I've seen before include "Make reduction" , "Decrease, lessen" , "Diet" , "get shorter" , "Make or become smaller" .)