Short maxim about part of sundial (6)
I believe the answer is:
gnomon
'part of sundial' is the definition.
(piece of a sundial)
'short maxim about' is the wordplay.
'short' means to remove the last letter.
'maxim' becomes 'gnome' (gnome is a kind of maxim).
'about' becomes 'on' (as in a book 'on' a certain topic).
'gnome' with its final letter taken off is 'gnom'.
'gnom'+'on'='GNOMON'
(Other definitions for gnomon that I've seen before include "Part of a sundial that casts the shadow" , "Pointer on a sundial" , "Part of a sundial projecting a shadow" , "Sundial shadow projector" , "Pin of a sundial whose shadow points to the hour" .)