Speak up, but be too clever for success
I believe the answer is:
out with it
'speak up' is the definition.
(I've seen this before)
'be too clever for success' is the wordplay.
'be too clever for' becomes 'outwit' (I've seen this before).
'success' becomes 'hit' (hit is a kind of success).
'outwit'+'hit'='OUT WITH IT'
'but' is the link.
(Other definitions for out with it that I've seen before include "divulge information immediately!" , "Squeal" , "Say what you think" , "Speak up" , "Demand for comment" .)