Real problem concealing lord's last tree (5)
I believe the answer is:
alder
'tree' is the definition.
(I know that alder is a type of tree)
'real problem concealing lord's last' is the wordplay.
'problem' indicates an anagram.
'concealing' indicates putting letters inside.
'last' says to take the final letters.
The final letter of 'lord' is 'd'.
'real' anagrammed gives 'aler'.
'aler' going around 'd' is 'ALDER'.
(Other definitions for alder that I've seen before include "Water resistant wood" , "Forward movement" , "Artificial fishing fly" , "Native tree" , "North temperate tree with cone-like fruit" .)