Success involves familiar wait (4,2)
I believe the answer is:
hold it
'wait' is the definition.
(I've seen this before)
'success involves familiar' is the wordplay.
'success' becomes 'hit' (a hit is a commercial success).
'involves' indicates putting letters inside.
'familiar' becomes 'old' (old can mean familiar or traditional).
'hit' going around 'old' is 'HOLD IT!'.
(Other definitions for hold it that I've seen before include "Hang on a minute" , "Stop right there!" , "Cease action - get a grip!" , "Wait" , "Informally, cease action" .)