Four astute tradesmen's stocks (6)
I believe the answer is:
tetrad
'four' is the definition.
(synonyms)
'astute tradesmen's stocks' is the wordplay.
'stocks' indicates the answer is hidden within the clue.
'TETRAD' is hidden in the letters of 'astute tradesmens'.
(Other definitions for tetrad that I've seen before include "quartet" , "Group or set of four" , "number of Gospel writers" .)