Deafening, behold, and extremely unbalanced (4)
I believe the answer is:
loud
'deafening' is the definition.
(similar in meaning)
'behold and extremely unbalanced' is the wordplay.
'behold' becomes 'lo' ('lo' is an archaic exclamation meaning 'look!').
'and' says to put letters next to each other.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'unbalanced' with its middle removed is 'ud'.
'lo'+'ud'='LOUD'
(Other definitions for loud that I've seen before include "vulgar" , "Blaring" , "Noisily showy?" , "strongly audible" , "Noisy or garish" .)