Upset by a stupid person, tattles now and then (7)
I believe the answer is:
agitate
'upset' is the definition.
(agitating is a kind of upseting)
'a stupid person tattles now and then' is the wordplay.
'stupid person' becomes 'git' (I can't justify this - if you can you should believe this answer much more).
'now and then' means one should take alternating letters (letters taken on a regular basis, now and then).
The alternating letters of 'tattles' are 'ate'.
'a'+'git'+'ate'='AGITATE'
'by' is the link.
Can you help me to learn more?
(Other definitions for agitate that I've seen before include "cause trouble" , "Disturb or shake" , "Disturb or churn" , "Stir; arouse" , "'Stir, excite (7)'" .)