They disturb and get a number up in arms (6)
I believe the answer is:
alarms
'they disturb and' is the definition.
The definition suggests an adverb but the answer is not.
'a number up in arms' is the wordplay.
'number' becomes 'l' (L is an example).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'in' means one lot of letters goes inside another.
'a'+'l'='al'
'al' in reverse letter order is 'la'.
'la' placed into 'arms' is 'ALARMS'.
'get' is the link.
Can you help me to learn more?
(Other definitions for alarms that I've seen before include "Frightens or alerts" , "Scares or startles" , "rattles" , "Warning signals" , "Frightens and startles" .)