Radical message, not the first, going to engineers (7)
I believe the answer is:
'radical' is the definition.
(radical can mean revolutionary or extreme)
'message not the first going to engineers' is the wordplay.
'message' becomes 'text' (text message).
'not the first' means to remove the first letter.
'going to' is a charade indicator (letters next to each other).
'engineers' becomes 'reme' (Royal Electrical and Mechanical Engineers).
'text' with its first letter removed is 'ext'.
(Other definitions for extreme that I've seen before include "Point furthest away" , "Of the greatest intensity" , "end" , "lacking moderation" , "Drastic -- limit" .)