Unreasonable adult dog biting character from Athens (1,3,4)
I believe the answer is:
a bit much
'unreasonable' is the definition.
(I've seen this before)
'adult dog biting character from athens' is the wordplay.
'adult' becomes 'a' (abbreviation).
'dog' becomes 'bitch' (I've seen this before).
'biting' indicates putting letters inside (some letters bite or swallow others).
'character from athens' becomes 'mu' (letter of the Greek alphabet).
'bitch' enclosing 'mu' is 'bitmuch'.
'a'+'bitmuch'='A BIT MUCH'
(Other definitions for a bit much that I've seen before include "Butch ami may be excessive, beyond the limit" , "That's not on" , "Taking liberties" , "out of order" , "unfair" .)