Article with strap in it (7)
I believe the answer is:
therein
'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).
'the'+'rein'='THEREIN'
(Other definitions for therein that I've seen before include "Inside that" , "Neither (anag) - into that" , "in that respect" , "enclosed by that place?" .)