Oil wood on top of mantelpiece (6)
I believe the answer is:
balsam
'oil' is the definition.
(thesaurus)
'wood on top of mantelpiece' is the wordplay.
'wood' becomes 'balsa' (balsa is a kind of wood).
'on top of' indicates taking the first letters (I've seen 'on top' mean this).
The initial letter of 'mantelpiece' is 'm'.
'balsa'+'m'='BALSAM'
(Other definitions for balsam that I've seen before include "Mal, BAs can produce a healing agent" , "Fragrant medicinal plant resin" , "(Aromatic medicine from) shrub" , "Aromatic ointment" , "Fragrant resin used in medicine and perfumes" .)