Behold, start of week makes you depressed (3)
I believe the answer is:
low
'depressed' is the definition.
(similar in meaning)
'behold start of week' is the wordplay.
'behold' becomes 'lo' ('lo' is an archaic exclamation meaning 'look!').
'start of' indicates taking the first letters.
The initial letter of 'week' is 'w'.
'lo'+'w'='LOW'
'makes you' acts as a link.
(Other definitions for low that I've seen before include "something from Guernsey?" , "Inferior, cow's call" , "Without great height" , "Opposite of high" , "Not elevated, may be base" .)