Leaders of think tank penning little message (5)
I believe the answer is:
tweet
'message' is the definition.
(I've seen this before)
'leaders of think tank penning little' is the wordplay.
'leaders of' says to take the initial letters.
'penning' is an insertion indicator.
'little' becomes 'wee' ('wee' can be similar in meaning to 'little'**).
The first letters of 'think tank' is 'tt'.
'tt' going around 'wee' is 'TWEET'.
(Other definitions for tweet that I've seen before include "online comment" , "Communicate" , "Short online message" , "Small bird's chirp" , "Short message posted on a social network" .)