Drink fuddled old boy, joining wrong army (6,4)
I believe the answer is:
bloody mary
'drink' is the definition.
(Bloody Mary is an example)
'fuddled old boy joining wrong army' is the wordplay.
'fuddled' is an anagram indicator.
'joining' means one lot of letters go next to another.
'wrong' is an anagram indicator.
'old'+'boy'='oldboy'
'oldboy' is an anagram of 'bloody'.
'army' with letters rearranged gives 'mary'.
'bloody'+'mary'='BLOODY MARY'
(Other definitions for bloody mary that I've seen before include "Vodka-based cocktail" , "The Red Queen" , "something to drink" , "Queen regarded unfavourably" , "Tomato juice and vodka" .)