Pig therefore knew, at the end (3)
I believe the answer is:
sow
'pig' is the definition.
(I know that sow is a type of pig)
'therefore knew at the end' is the wordplay.
'therefore' becomes 'so' ('so' can mean 'consequently' or 'therefore').
'at the end' suggests the final letters.
The last letter of 'knew' is 'w'.
'so'+'w'='SOW'
(Other definitions for sow that I've seen before include "Scatter seed; pig" , "swine" , "Plant seeds for the pig" , "Female pig" , "Female bear" .)