Middle of battle interrupted by little message (5)
I believe the answer is:
tweet
'message' is the definition.
(type of online message)
'middle of battle interrupted by little' is the wordplay.
'middle of' indicates the central letters.
'interrupted by' is an insertion indicator.
'little' becomes 'wee' (similar in meaning).
The middle letters of 'battle' are 'tt'.
'tt' going around 'wee' is 'TWEET'.
(Other definitions for tweet that I've seen before include "Online communication" , "Short message on a social medium" , "Chirp of a small bird" , "Note of a small bird" , "little message" .)