Rats running hard at rubbish (5)
I believe the answer is:
trash
'rubbish' is the definition.
(I know that rubbish can be written as trash)
'rats running hard' is the wordplay.
'running' indicates an anagram.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'rats' with letters rearranged gives 'tras'.
'tras'+'h'='TRASH'
'at' is the link.
(Other definitions for trash that I've seen before include "Waste material (US)" , "bunk" , "Junk" , "Rubbish, litter" , "bull" .)