An example of this would be The Thing (7)
I believe the answer is:
article
'an' is the definition.
('an' is an indefinite article in English)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for article that I've seen before include "Maybe the" , "Division" , "thing required" , "Rile cat (anag.)" , "to cause amusement, say, to a king" .)