Clump, cluster (4)
I believe the answer is:
knot
I believe this is a double definition.
'clump' is the first definition.
(I know that knot is a type of clump)
'cluster' is the second definition.
(I know that knot is a type of cluster)
(Other definitions for knot that I've seen before include "Nautical measure" , "To secure < > bird" , "Tight interlacing of cord, string etc" , "Eg, hitch" , "Fastening - unit of speed" .)