Wobbly: lacking capacity to cross street (8)
I believe the answer is:
unstable
'wobbly' is the definition.
('unstable' can be similar in meaning to 'wobbly')
'lacking capacity to cross street' is the wordplay.
'lacking capacity' becomes 'unable'.
'to cross' indicates putting letters inside.
'street' becomes 'st'.
'unable' enclosing 'st' is 'UNSTABLE'.
(Other definitions for unstable that I've seen before include "Moody" , "Subject to change, variable" , "Rickety" , "not reliable" , "not settled" .)