Duller month in Mediterranean city (7)
I believe the answer is:
palermo
'mediterranean city' is the definition.
'duller month' is the wordplay.
'duller' becomes 'paler'.
'month' becomes 'mo' (abbreviation for month).
'paler'+'mo'='PALERMO'
'in' acts as a link.
(Other definitions for palermo that I've seen before include "ity of Italy" , "Chief city of Sicily" , "Sicilian capital" , "Sicilian city" , "Italian city" .)