A weed to pull to pieces, we hear
I believe the answer is:
tare
'a weed' is the definition.
(I've seen this before)
'pull to pieces we hear' is the wordplay.
'pull to pieces' becomes 'tear' (I've seen this in another clue).
'we hear' shows a homophone (sound like).
'tear' is a homophone of 'TARE'.
'to' acts as a link.
(Other definitions for tare that I've seen before include "Moved very quickly" , "Weedy annual grass, often occurs in grainfields" , "Variety of vetch" , "Vetch plant" , "Weight of unloaded vehicle" .)