Lad sent queen a tree (5)
I believe the answer is:
alder
'a tree' is the definition.
(alder is a kind of tree)
'lad sent queen' is the wordplay.
'sent' is an anagram indicator (send can mean to excite or stimulate).
'queen' becomes 'er' (abbreviation for Elizabeth Regina**).
'lad' with letters rearranged gives 'ald'.
'ald'+'er'='ALDER'
(Other definitions for alder that I've seen before include "Tree or shrub of the birch family" , "One [TYPE OF TREE]" , "Tree or shrub whose wood is resistant to underwater rot" , "North temperate tree with cone-like fruit" , "Tree with catkins" .)