Judge grounds holding one million (8)
I believe the answer is:
estimate
'judge' is the definition.
(I've seen this before)
'grounds holding one million' is the wordplay.
'grounds' becomes 'estate' (I've seen this before**).
'holding' means one lot of letters goes inside another.
'one million' becomes 'im' (I've seen this in other clues).
'estate' going around 'im' is 'ESTIMATE'.
(Other definitions for estimate that I've seen before include "'Judge, calculate (8)'" , "Reckon value" , "approximate price" , "Make approximate judgement of size or amount" , "Statement of expected cost" .)