Gem that's mostly white or pink with green sides (5)
I believe the answer is:
pearl
'gem that's mostly white or pink' is the definition.
(white or pink gem)
'with green sides' is the wordplay.
'with' says to put letters next to each other.
'green' becomes 'pea green' (pea green is a kind of green).
'sides' becomes 'RL' (abbreviations for left and right).
'pea' put next to 'rl' is 'PEARL'.
(Other definitions for pearl that I've seen before include "Gem; an attacked Harbor" , "Off-white colour" , "Small type" , "Girl < , pretty" , "prized example" .)