Work that’s involved soup (4)
I believe the answer is:
opus
'work' is the definition.
(I've seen this before)
'involved soup' is the wordplay.
'involved' indicates anagramming the letters (involved can mean confusing or complex).
'soup' anagrammed gives 'OPUS'.
'that's' acts as a link.
(Other definitions for opus that I've seen before include "Artistic work" , "Soup (anag.)" , "Major work" , "A Latin work" , "Composer's work" .)