Abstainer penning little message (5)
I believe the answer is:
tweet
'message' is the definition.
(I've seen this before)
'abstainer penning little' is the wordplay.
'abstainer' becomes 'tt' (I've seen this in other clues).
'penning' indicates putting letters inside.
'little' becomes 'wee' ('wee' can be similar in meaning to 'little').
'tt' enclosing 'wee' is 'TWEET'.
(Other definitions for tweet that I've seen before include "little message" , "throwaway message?" , "Short message on a social medium" , "Sound of small bird" , "Communicate" .)