Also mount up slowly (7)
I believe the answer is:
andante
'slowly' is the definition.
(musical term)
'also mount up' is the wordplay.
'also' becomes 'and' (both can mean in addition).
'mount' becomes 'etna' (Etna is an example).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'etna' in reverse letter order is 'ante'.
'and'+'ante'='ANDANTE'
(Other definitions for andante that I've seen before include "Unhurried pace" , "moving unhurriedly" , "Fairly slow (mus.)" , "(Musically) at a moderate pace" , "Slow, even time (mus.)" .)