Mount up and trot after the animal (8)
I believe the answer is:
antelope
'the animal' is the definition.
(I've seen this before)
'mount up and trot' is the wordplay.
'mount' becomes 'etna' (Etna is an example).
'up' is a reversal indicator (in down clue: letters go upwards).
'and' is a charade indicator (letters next to each other).
'trot' becomes 'lope' (synonyms).
'etna' back-to-front is 'ante'.
'ante'+'lope'='ANTELOPE'
'after' acts as a link.
(Other definitions for antelope that I've seen before include "One petal for this animal" , "Fast runner" , "grazing animal" , "Eg, gazelle or gnu" , "Gazelle or impala" .)