The answer is...
Fish trap backward. Two handfuls mock.
def "fish trap" (GAR NET), and backward "TEN RAG" (ten: 2 times 5 fingers - thanks, Jon!)
River in a car, a spice and a rock.
G(ARNE)T; def "spice and a rock" cinnamon stone is a type of garnet
Gemstone in sugar, nethermost...
suGAR NEThermost; def "Gemstone"
...shade. The alien called, all disarrayed.
def "shade" (garnet is a shade of red); ET RANG*
GENTLEPURPLERAIN anagrammed is LIP PEN LURE GARNET.
And the title hides the word
Pyrope, a type of garnet.