Dog without lead is an irritation (4)
I believe the answer is:
itch
'an irritation' is the definition.
(an itch irritates you)
'dog without lead' is the wordplay.
'dog' becomes 'bitch' (I've seen this before).
'without lead' means to remove the first letter.
'bitch' with its first letter taken off is 'ITCH'.
'is' acts as a link.
(Other definitions for itch that I've seen before include "are dying" , "A strong restless desire" , "Strong desire to do something" , "Nagging wish" , "following seven years in union?" .)