Messages me, upset over pains (6)
I believe the answer is:
emails
'messages' is the definition.
(emails are electronic messages)
'me upset over pains' is the wordplay.
'upset over' shows that the letters should be reversed in order (knocked over).
'pains' becomes 'ails' ('ail' can be a synonym of 'pain').
'me' in reverse letter order is 'em'.
'em'+'ails'='EMAILS'
(Other definitions for emails that I've seen before include "Electronic communications" , "Computer communications" , "Electronic messages" .)