Gelatinous substance a scrap of cloth picked up (4)
I believe the answer is:
agar
'gelatinous substance' is the definition.
(I've seen this in another clue)
'a scrap of cloth picked up' is the wordplay.
'scrap of cloth' becomes 'rag' (I've seen this before).
'picked up' says the letters should be written backwards.
'rag' written backwards gives 'gar'.
'a'+'gar'='AGAR'
(Other definitions for agar that I've seen before include "What comes out of seaweed" , "Jelly medium" , "Food-thickening jelly" , "a newspaper rejected" , "Jelly got from seaweeds, used in foods" .)