Powerless to cross street to get bananas (8)
I believe the answer is:
unstable
'bananas' is the definition.
(I know that bananas can informally mean mentally ill)
'powerless to cross street' is the wordplay.
'powerless' becomes 'unable' (similar in meaning).
'to cross' is an insertion indicator.
'street' becomes 'st' (abbreviation).
'unable' enclosing 'st' is 'UNSTABLE'.
'to get' acts as a link.
(Other definitions for unstable that I've seen before include "Wobbly" , "insecure" , "Ban lutes (anag.)" , "Not steady or dependable" , "not settled" .)