An effect of drinking gallons in German city (8)
I believe the answer is:
hangover
'an effect of drinking' is the definition.
(drinking alcohol causes hangovers)
'gallons in german city' is the wordplay.
'gallons' becomes 'G' (abbreviation).
'in' means one lot of letters goes inside another.
'german city' becomes 'hanover' (city in Germany).
'g' put within 'hanover' is 'HANGOVER'.
(Other definitions for hangover that I've seen before include "problem arising?" , "Morning-after problem" , "result of over-indulgence?" , "Effect of previous day's alcohol" , "After-effect of drinking spree" .)