Eccentric old boy and girl must get a drink (6,4)
I believe the answer is:
bloody mary
'a drink' is the definition.
(Bloody Mary is an example)
'eccentric old boy and girl' is the wordplay.
'eccentric' is an anagram indicator.
'and' says to put letters next to each other.
'girl' becomes 'mary' (**).
'old'+'boy'='oldboy'
'oldboy' with letters rearranged gives 'bloody'.
'bloody'+'mary'='BLOODY MARY'
'must get' is the link.
(Other definitions for bloody mary that I've seen before include "The Red Queen" , "Tomato juice and vodka" , "killing Queen?" , "Nickname of Henry VIII's first daughter" , "something to drink" .)