Animals taking exercise further down (6)
I believe the answer is:
deeper
'further down' is the definition.
(I've seen this before)
'animals taking exercise' is the wordplay.
'animals' becomes 'deer'.
'taking' indicates putting letters inside.
'exercise' becomes 'pe' (Physical Education).
'deer' enclosing 'pe' is 'DEEPER'.
(Other definitions for deeper that I've seen before include "More profound or more intense" , "Lower in pitch" , "more involved" , "Profounder" , "Wiser" .)