Really bad from City: Real Betis initially fantastic (9)
I believe the answer is:
execrable
'really bad' is the definition.
(I know this)
'from city real betis initially fantastic' is the wordplay.
'from' becomes 'ex' ('from' in Latin).
'city' becomes 'EC' (the City of London postcode).
'initially' indicates taking the first letters.
'fantastic' indicates an anagram (a fantastic or improbable ordering of the letters).
The first letter of 'betis' is 'b'.
'real'+'b'='realb'
'realb' anagrammed gives 'rable'.
'ex'+'ec'+'rable'='EXECRABLE'
(Other definitions for execrable that I've seen before include "Appallingly bad" , "Extremely bad or unpleasant" , "Loathsome" , "Woeful" , "A celeb rex may be abominable" .)