Source of oil on end of mantelpiece, alongside source of milk (3,4)
I believe the answer is:
tea tree
'source of oil' is the definition.
(source of tea tree oil)
'on end of mantelpiece alongside source of milk' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'end of' says to take the final letters.
'alongside' is a charade indicator (letters next to each other).
'source of milk' becomes 'teat' (body part which dispenses milk).
The final letter of 'mantelpiece' is 'e'.
're'+'e'='ree'
'ree' after 'teat' is 'TEA-TREE'.
(Other definitions for tea tree that I've seen before include "source of antiseptic" , "Rosie's origin" , "plant down under" , "antiseptic producer" , "Australian shrub" .)