Article with strap in it (7)
I believe the answer is:
Here is my best explanation:
'in it' is the definition.
'article with strap' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech).
'with' says to put letters next to each other.
'strap' becomes 'rein' (I've seen this before).
(Other definitions for therein that I've seen before include "enclosed by that place?", "Neither (anag) - into that", "Inside that", "In that place" and "in that respect".)