A filling sort of paste (5)
I believe the answer is:
putty
'paste' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for putty that I've seen before include "Glazier's paste" , "Malleable form of glas holder" , "Window-sealing paste" , "Window fixer" , "Whiting and linseed oil paste" .)