It's shorter when followed by a dog (9)
I believe the answer is:
curtailed
'it's shorter' is the definition.
(thesaurus)
'followed by a dog' is the wordplay.
'followed' becomes 'tailed' (I've seen this in another clue).
'by' is a charade indicator (letters next to each other).
'a dog' becomes 'cur' (a cur is an aggressive or scruffy dog).
'tailed' put after 'cur' is 'CURTAILED'.
'when' is the link.
(Other definitions for curtailed that I've seen before include "docked" , "Cut short" , "Broken off" , "Clipped" , "limited" .)