Not even the best hard cogs (5)
I believe the answer is:
teeth
'cogs' is the definition.
(I've seen this in another clue)
'not even the best hard' is the wordplay.
'not even' indicates alternate letters (remove even-numbered letters).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'the'+'best'='thebest'
The alternate letters of 'thebest' are 'teet'.
'teet'+'h'='TEETH'
(Other definitions for teeth that I've seen before include "Fangs" , "Force v" , "One may be armed to them" , "enforcement mechanisms" , "Gearwheel projections" .)