Flower lit up rocks (5)
I believe the answer is:
tulip
'flower' is the definition.
(thesaurus)
'lit up rocks' is the wordplay.
'rocks' indicates anagramming the letters.
'lit'+'up'='litup'
'litup' anagrammed gives 'TULIP'.
(Other definitions for tulip that I've seen before include "cause of mania once" , "It blooms in 11 down" , "... and another [FLOWER]" , "Plant of the lily family with a single showy flower" , "Flowering bulb (from Amsterdam?)" .)