Dull city centre's gone flashy inside (6)
I believe the answer is:
cloudy
'dull' is the definition.
(I've seen this before)
'city centre's gone flashy inside' is the wordplay.
'centre's gone' means to remove the middle letters.
'flashy' becomes 'loud' (synonyms).
'inside' indicates putting letters inside.
'city' with its middle removed is 'cy'.
'cy' going around 'loud' is 'CLOUDY'.
(Other definitions for cloudy that I've seen before include "'Overcast, nebulous (6)'" , "(Of a liquid) not clear" , "Murky or nebulous" , "Not transparent" , "Dull, overcast" .)