Holm oak (4)
I believe the answer is:
ilex
'holm oak' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ilex that I've seen before include "'Tree group, includes holly (4)'" , "Holly, classically" , "Holly, perhaps" , "Genus of trees that includes hollies" , "The holm-oak" .)