Sweetheart, I cleaned inside of thing (7)
I believe the answer is:
article
'thing' is the definition.
(I've seen this before)
'sweetheart i cleaned inside' is the wordplay.
'inside' says the answer is hidden in the clue.
'ARTICLE' can be found hidden inside 'sweetheart i cleaned'.
'of' acts as a link.
(Other definitions for article that I've seen before include "Possibly the" , "It may be the" , "Commodity" , "It could be the" , "Recital (anag.)" .)