Ferret to graze on food underground (4,4)
I believe the answer is:
root crop
'food underground' is the definition.
(food found underground)
'ferret to graze on' is the wordplay.
'ferret' becomes 'root' (as in rooting or ferreting around for something).
'to graze' becomes 'crop' ('crop' can be a synonym of 'graze').
'on' means one lot of letters go next to another.
'root'+'crop'='ROOT-CROP'
(Other definitions for root crop that I've seen before include "Swedes, say" , "Vegetable" , "Potatoes or carrots" , "Eg, potatoes" , "Tuber, beet, etc" .)