Finding out Edward returned, etc., mistakenly in grand opening (9)

'finding' is the definition.
('detect' can be a synonym of 'find')

'out edward returned etc mistakenly in grand opening' is the wordplay.
'out' means one lot of letters goes inside another.
'edward' becomes 'ted' ().
'returned' shows that the letters should be reversed in order.
'mistakenly' is an anagram indicator.
'opening' suggests taking the first letters.
The initial letter of 'grand' is 'g'.
'ted' backwards is 'det'.
'etc' with letters rearranged gives 'tec'.
'det' going around 'tec' is 'detect'.
'detect'+'in'+'g'='DETECTING'