Return of goods — deficiency ultimately unclear (5)
I believe the answer is:
'unclear' is the definition.
'return of goods deficiency ultimately' is the wordplay.
'return' says the letters should be written backwards.
'goods' becomes 'gg' (I've seen this in other clues).
'ultimately' indicates one should take the final letters.
The final letter of 'deficiency' is 'y'.
'of' back-to-front is 'fo'.
(Another definition for foggy that I've seen is " Very misty".)