Lithium crystals initially applied to birds or plants (7)
I believe the answer is:
lichens
'plants' is the definition.
(lichen is a kind of plant)
'lithium crystals initially applied to birds' is the wordplay.
'lithium' becomes 'Li' (chemical symbol).
'initially' indicates taking the first letters.
'applied to' is a charade indicator (letters next to each other).
'birds' becomes 'hens' (hen is a type of bird).
The first letter of 'crystals' is 'c'.
'li'+'c'+'hens'='LICHENS'
'or' acts as a link.
(Another definition for lichens that I've seen is " Tiny grey and green plant growths on rocks and tree-trunks".)