Message me over trouble (5)
I believe the answer is:
'message' is the definition.
(email is a type of message)
'me over trouble' is the wordplay.
'over' is a reversal indicator.
'trouble' becomes 'ail' (synonyms).
'me' in reverse letter order is 'em'.
'em'+'ail'='EMAIL'
(Other definitions for email that I've seen before include "Message sent via computer" , "Electronic communication" , "Messages via the internet" , "Post" , "Digital message" .)