Sounds unhappy about the said magnitude (5)
I believe the answer is:
sighs
'sounds' is the definition.
(sigh is a kind of sound)
'said magnitude' is the wordplay.
'said' shows a homophone (sound like).
'magnitude' becomes 'size' (size is a kind of magnitude).
'size' sounds like 'SIGHS'.
'unhappy about the' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for sighs that I've seen before include "gentle sounds" , "Yearns" , "Moans" , "Bridge of - - (Venice)" , "Sounds of weariness" .)