Also mount up slowly (7)
I believe the answer is:
Here is my best explanation:
'slowly' is the definition.
Both the definition and answer are adverbs. Maybe you can see an association between them that I can't see?
'also mount up' is the wordplay.
'also' becomes 'and' (I've seen this before).
'mount' becomes 'etna' (Etna is an example**).
'up' is a reversal indicator (in down clue: letters go upwards).
'etna' reversed gives 'ante'.
(Other definitions for andante that I've seen before include "Moderately slow movement", "(Of tempo) moderately slow", "quite slow", "Fairly slow movement", "Slow, even time (mus.)", "Movement", "at a gentle pace", "moving steadily", "Moderately slow tempo" and "deliberately played".)