Flask, article Mrs Overall initially used (7)
I believe the answer is:
thermos
'flask' is the definition.
(thermos is a kind of flask)
'article mrs overall initially used' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech**).
'initially' suggests taking the first letters.
'used' indicates anagramming the letters.
The initial letter of 'overall' is 'o'.
'mrs'+'o'='mrso'
'mrso' with letters rearranged gives 'rmos'.
'the'+'rmos'='THERMOS'
(Other definitions for thermos that I've seen before include "Vacuum flask for mothers" , "Flask trade mark" , "Container to retain heat" , "Heat-retaining flask for mothers" .)