Short dog with a little tail (4)
I believe the answer is:
curt
'short' is the definition.
(impolite)
'dog with a little tail' is the wordplay.
'dog' becomes 'cur' (a cur is an aggressive or scruffy dog).
'with' says to put letters next to each other.
'a little' says to take the initial letters.
The initial letter of 'tail' is 't'.
'cur'+'t'='CURT'
(Other definitions for curt that I've seen before include "Brief and terse" , "Brusque, short" , "Snappy" , "Brusque, abrupt in speech" , "Concise" .)