Concise expression (4)
I believe the answer is:
pith
'concise expression' is the definition.
This is all the clue.
(Other definitions for pith that I've seen before include "nub of matter" , "Tissue inside the rind of citrus fruits" , "Soft tissue; essence" , "Central part" , "Spongy tissue lining rinds of oranges, or the essential part" .)