Back up in thus producing work (4)
I believe the answer is:
opus
'work' is the definition.
(an opus is an artistic work)
'back up in thus' is the wordplay.
'back' says the letters should be written in reverse.
'in' means one lot of letters goes inside another.
'thus' becomes 'so' (synonyms).
'up' inserted inside 'so' is 'supo'.
'supo' written backwards gives 'OPUS'.
'producing' is the link.
(Other definitions for opus that I've seen before include "Work (mus.)" , "Piece of music (with number)" , "Major work" , "Soup (anag.)" , "Musical piece" .)