Diminutive figure briefly working, bit of an old timer (6)
I believe the answer is:
gnomon
'bit of an old timer' is the definition.
(part of a sundial - something that tells the time)
'diminutive figure briefly working' is the wordplay.
'diminutive figure' becomes 'gnome' (gnomes are imaginary small creatures).
'briefly' means to remove the last letter.
'working' becomes 'on' (on can mean working or activated).
'gnome' with its last letter removed is 'gnom'.
'gnom'+'on'='GNOMON'
(Other definitions for gnomon that I've seen before include "Part of a sundial projecting a shadow" , "Part of a sundial that casts the shadow" , "Pointer on a sundial" , "where time casts its shadow" , "Pin of a sundial whose shadow points to the hour" .)