Gelatinous substance from a river crossing Georgia (4)
I believe the answer is:
agar
'gelatinous substance' is the definition.
(I've seen this before)
'a river crossing georgia' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'georgia' becomes 'ga' (abbreviation for the US state).
'a'+'r'='ar'
'ar' enclosing 'ga' is 'AGAR'.
'from' acts as a link.
(Other definitions for agar that I've seen before include "Food thickening agent obtained from seaweed" , "used in making soups?" , "contribution to culture?" , "Food thickener (derived from seaweed)" , "found in the sea" .)