I bore a well — its first litre (3)
I believe the answer is:
awl
'i bore' is the definition.
(tool for boring holes)
'a well its first litre' is the wordplay.
'its first' indicates taking the first letters.
'litre' becomes 'L' (abbreviation).
The initial letter of 'well' is 'w'.
'a'+'w'+'l'='AWL'
(Other definitions for awl that I've seen before include "One piercing" , "Tool for punching small holes" , "Tool for hole making" , "Boring tool sounds comprehensive" , "Carpenter's tool" .)