Pine relative (3)
'pine relative' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for fir that I've seen before include "Tree for an Irishman" , "Evergreen conifer" , "Evergreen tree for Irish men?" , "Kind of cone-bearing tree" , "wood" .)
(Other definitions for ache that I've seen before include "Nagging pain" , "burn" , "painful" , "Longing" , "Each (anag.)" .)