Frothy stuff in bird's hole in the ground (6,4)
I believe the answer is:
cuckoo spit
'frothy stuff' is the definition.
(cuckoo spit is a frothy substance)
'bird's hole in the ground' is the wordplay.
'bird's' becomes 'cuckoos' (cuckoo is a kind of bird).
'hole in the ground' becomes 'pit' (pit is a kind of hole in the ground).
'cuckoos'+'pit'='CUCKOO-SPIT'
'in' acts as a link.
(Other definitions for cuckoo spit that I've seen before include "Protective froth secreted by froghopper larvae" , "affording protection for insects" .)