Article on small, extremely costly, tool (6)
I believe the answer is:
scythe
'tool' is the definition.
(scythe is a kind of tool)
'article on small extremely costly' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech**).
'on' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'extremely' means to remove the middle letters.
'costly' with its middle removed is 'cy'.
's'+'cy'='scy'
'the' after 'scy' is 'SCYTHE'.
(Other definitions for scythe that I've seen before include "Grain-cutting tool" , "Manual hay-cutter wielded with both hands" , "Old hay cutter with long handle and curved blande" , "Reaper's cutter" , "Manual grass cutter" .)