Source of milk giving us energy when drunk (8)
I believe the answer is:
'source of milk' is the definition.
(I've seen this before)
'giving us energy when drunk' is the wordplay.
'giving' says to put letters next to each other.
'when drunk' indicates an anagram (a drunken or disorderly form of the letters).
'us'+'energy' is 'usenergy'.
'usenergy' anagrammed gives 'GUERNSEY'.
(Other definitions for guernsey that I've seen before include "Neighbour of Jersey" , "Lyric poem" , "One of the Channel Islands" , "Sir Walter Scott novel" , "Breed of cattle and a Channel Island" .)