A dog without lead is too much! (4)
I believe the answer is:
over
'too much' is the definition.
(over can mean to too great an extent)
'a dog without lead' is the wordplay.
'a dog' becomes 'rover' (Rover is a common name for a pet dog).
'without lead' means to remove the first letter.
'rover' with its first letter removed is 'OVER'.
'is' acts as a link.
(Other definitions for over that I've seen before include "See 18" , "Excessive" , "Higher than; finished" , "Period of play" , "A bowler's round in cricket" .)