Season half gone — sing, as one doing business (8)
I believe the answer is:
merchant
'one doing business' is the definition.
(person involved in trade)
'season half gone sing' is the wordplay.
'season' becomes 'summer' (summer is a season).
'half gone' indicates taking half.
'sing' becomes 'chant' (chanting is a kind of singing).
'summer' with half the letters taken is 'mer'.
'mer'+'chant'='MERCHANT'
'as' is the link.
(Other definitions for merchant that I've seen before include "Businessperson engaged in trade" , "Wholesale trader" , "Antonio, for one" , "that's often associated with speed" , "One involved in wholesale trade" .)