The beginnings of roots underneath the furrow (3)
I believe the answer is:
rut
'furrow' is the definition.
(rut is a kind of furrow)
'the beginnings of roots underneath the' is the wordplay.
'the beginnings of' says to take the initial letters (I've seen 'the beginning of' mean this).
The first letters of 'roots underneath the' is 'RUT'.
(Other definitions for rut that I've seen before include "Wheel-made furrow" , "Oestrus" , "Boring groove to be stuck in!" , "Well-worn track" , "Deep track or tedious way of life" .)