Heaps of small nails (6)
I believe the answer is:
stacks
'heaps' is the definition.
(stack is a kind of heap)
'small nails' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'nails' becomes 'tacks' (tack is a kind of nail).
's'+'tacks'='STACKS'
'of' is the link.
(Other definitions for stacks that I've seen before include "Orderly piles or heaps" , "Orderly heaps as of hay" , "Tons" , "bags?" , "Ricks" .)