Also mount up slowly (7)
I believe the answer is:
andante
'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'.
'and'+'ante'='ANDANTE'
Can you help me to learn more?
(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.)" .)