Fail to get this compiler's message (7)
I believe the answer is:
missive
'message' is the definition.
(I've seen this before)
'fail to get this compiler's' is the wordplay.
'fail to get' becomes 'miss' (I've seen this in another clue).
'this compiler's' becomes 'i've' ('I have' referring to the crossword setter).
'miss'+'ive'='MISSIVE'
(Other definitions for missive that I've seen before include "Letter" , "message" , "intelligence in this?" .)