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' (cur is a kind of 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" .)