Tree always seems huge, first of all (3)
I believe the answer is:
ash
'tree' is the definition.
(ash is a kind of tree)
'always seems huge first of all' is the wordplay.
'first of all' says to take the initial letters.
The initial letters of 'always seems huge' is 'ASH'.
(Other definitions for ash that I've seen before include "Rowan tree is also called the mountain . . ." , "Grey-barked tree" , "In fireplace it may be" , ". . . . [FOREST] One of which" , "Tree with pale wood" .)