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 before).
'by' means one lot of letters go next to another.
'a dog' becomes 'cur' (a cur is an aggressive or scruffy dog).
'tailed' 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" .)