Short dog is on time (4)
I believe the answer is:
curt
'short' is the definition.
('curt' can be a synonym of 'short')
'dog is on time' is the wordplay.
'dog' becomes 'cur' (cur is a kind of dog).
'is on' is a charade indicator (letters next to each other).
'time' becomes 't'.
'cur'+'t'='CURT'
(Other definitions for curt that I've seen before include "Brief and to the point" , "Brief and brusque" , "Dismissively short" , "Shrub with bright autumn colours" , "Brief, dismissive" .)