Winning in poshest ground open for business (3,2,4)
I believe the answer is:
set up shop
'ground open for business' is the definition.
I know nothing about this answer so I can't tell whether this works.
'winning in poshest' is the wordplay.
I cannot quite understand how this works, but
'winning' could be 'up' (eg being up at half time in football) and 'up' is found within the answer.
an anagram of 'poshest' is 'setshop' which is within the left over letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for set up shop that I've seen before include "begin operations" , "get market going" , "start operations" , "get business going" , "Begin trading" .)