The old Welsh conifer (3)
I believe the answer is:
yew
'conifer' is the definition.
(yew is a kind of conifer)
'the old welsh' is the wordplay.
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'welsh' becomes 'W' (abbreviation).
'ye'+'w'='YEW'
(Other definitions for yew that I've seen before include "This tree grows in churchyard" , "Evergreen tree with dark leaves and red berries" , "'Evergreen tree, reddish-brown wood (3)'" , "Conifer" , "Leaving body" .)