Part of the foliage of a maple, a fir, etc. (4)
'part of the foliage' is the definition.
'leaf' can be an answer for 'foliage' (synonyms). I am not sure about the 'part of the' bit.
'of a maple a fir etc' is the wordplay.
'of' says the answer is hidden in the clue.
'LEAF' is hidden within 'maple a fir etc'.
Can you help me to learn more?
(Other definitions for leaf that I've seen before include "Part of a plant and of a book", "Two pages of a book", "Bit of tree, of book", "In fall, one dislocated", "Hinged or removable table flap".)