Condition inside half of ground not declared (8)
I believe the answer is:
unstated
'not declared' is the definition.
(I've seen this before)
'condition inside half of ground' is the wordplay.
'condition' becomes 'state' (the state something is in).
'inside' means one lot of letters goes inside another.
'half of' indicates taking half.
'ground' with half the letters taken is 'und'.
'state' put within 'und' is 'UNSTATED'.
(Other definitions for unstated that I've seen before include "Not declared" , "taken for granted" , "understood" , "not mentioned" , "Not said" .)