More difficult check reviewed on god (8)
I believe the answer is:
thornier
'more difficult' is the definition.
('thorny' can be similar in meaning to 'difficult')
'check reviewed on god' is the wordplay.
'check' becomes 'rein' (both can mean to control or limit).
'reviewed' is a reversal indicator.
'on' means one lot of letters go next to another.
'god' becomes 'thor' (Thor is an example).
'rein' written backwards gives 'nier'.
'nier' put after 'thor' is 'THORNIER'.
(Another definition for thornier that I've seen is " More spiny, tricky (problem)".)