Unimaginative slogan of beer-minded Prohibitionist left unfinished (5)
I believe the answer is:
banal
'unimaginative' is the definition.
(thesaurus)
I don't understand the remainder of the clue.
(Other definitions for banal that I've seen before include "'Hackneyed, trite (5)'" , "Old hat" , "boringly dull" , "Stock" , "Lacking originality" .)