Deist running with friend in ballpark (9)
I believe the answer is:
estimated
'in ballpark' is the definition.
'deist running with friend' is the wordplay.
'running' indicates anagramming the letters.
'with' is an insertion indicator.
'friend' becomes 'mate' (mate is a kind of friend).
'deist' with letters rearranged gives 'estid'.
'estid' enclosing 'mate' is 'ESTIMATED'.
(Other definitions for estimated that I've seen before include "Calculated roughly" , "Judged roughly" , "Approximated, reckoned" , "Calculated approximately" , "Rough" .)