Rocky's incompetent boxing extremely suspect (8)
I believe the answer is:
unstable
'rocky's' is the definition.
(similar in meaning)
'incompetent boxing extremely suspect' is the wordplay.
'incompetent' becomes 'unable' (I've seen this before).
'boxing' means one lot of letters goes inside another ('box' can mean contain, as in to put in a box).
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'suspect' with its middle taken out is 'st'.
'unable' enclosing 'st' is 'UNSTABLE'.
(Other definitions for unstable that I've seen before include "insecure" , "not settled" , "Volatile, precarious" , "Wobbly" , "Not steady, volatile" .)