Articles are mostly bit of tosh, essentially (2,5)
I believe the answer is:
at heart
'essentially' is the definition.
(thesaurus)
'articles are mostly bit of tosh' is the wordplay.
'articles' becomes 'athe' (two examples of the part-of-speech: a+the**).
'mostly' means to remove the last letter.
'bit of' suggests taking the first letters.
The first letter of 'tosh' is 't'.
'are' with its last letter removed is 'ar'.
'athe'+'ar'+'t'='AT HEART'
(Other definitions for at heart that I've seen before include "Fundamentally" , "Basically" , "In one's real nature - a threat (anag)" , "Really" , "Substantially" .)