Tree in Sellafirth (3)
I believe the answer is:
fir
'tree' is the definition.
(fir tree is a kind of tree)
'in sellafirth' is the wordplay.
'in' says the answer is hidden in the clue.
'FIR' is hidden in the letters of 'sellafirth'.
(Other definitions for fir that I've seen before include "Evergreen conifer" , "Tree for an Irishman" , "Coniferous tree" , "Tree for Irishmen" , "Conifer tree" .)