A large red sort of shrub (5)
I believe the answer is:
alder
'shrub' is the definition.
(thesaurus)
'a large red sort' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'sort' indicates anagramming the letters.
'red' with letters rearranged gives 'der'.
'a'+'l'+'der'='ALDER'
'of' is the link.
(Other definitions for alder that I've seen before include "Tree related to the birch" , "Tree whose bark is used in tanning" , "Water resistant wood" , "One [TYPE OF TREE]" , "New Forest resident" .)