Jelly runs under the stove (4)
I believe the answer is:
agar
'jelly' is the definition.
(gelatinous substance)
'runs under the stove' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'under' says to put letters next to each other.
'the stove' becomes 'aga' (I've seen this before).
'r' put after 'aga' is 'AGAR'.
(Other definitions for agar that I've seen before include "found in the sea" , "Medium for growth" , "used in making soups?" , "Jelly used in bacteria culture" , "a newspaper rejected" .)