One sitting in car arrested by copper’s careful (8)
I believe the answer is:
cautious
'careful' is the definition.
('cautious' is associated in meaning with 'careful')
'one sitting in car arrested by copper's' is the wordplay.
'one' becomes 'i' (Roman numeral).
'sitting in' indicates putting letters inside.
'car' becomes 'auto' ('auto' can be a synonym of 'car').
'arrested by' indicates putting letters inside (I've seen this in other clues).
'copper's' becomes 'cus' (chemical symbol).
'i' inserted within 'auto' is 'autio'.
'autio' placed into 'cus' is 'CAUTIOUS'.
(Other definitions for cautious that I've seen before include "'Watchful, prudent (8)'" , "Unadventurous" , "Careful, shrewd" , "Careful to avoid dangers" , "safe" .)