Mind; beer foam (4)
I believe the answer is:
head
I believe this clue is a double definition.
'mind' is the first definition.
(I know that mind can be written as head)
'beer foam' is the second definition.
(I've seen this in another clue)
(Other definitions for head that I've seen before include "Cranium" , "River or stream source" , "Froth on beer" , "Person in charge" , "See 25" .)