Briefly had success — they are about to retire (3,3,3)
I believe the answer is:
hit the hay
'retire' is the definition.
(synonyms)
'briefly had success they are about' is the wordplay.
'briefly' means to remove the last letter.
'success' becomes 'hit' (a hit is a commercial success).
'are about' means one lot of letters goes inside another.
'had' with its last letter removed is 'ha'.
'hit'+'they'='hitthey'
'ha' placed into 'hitthey' is 'HIT THE HAY'.
'to' acts as a link.
(Other definitions for hit the hay that I've seen before include "Turn in" , "Crash" , "Go to bed (informal)" , "(retired)" , "Retire to bed (informal)" .)