Switch lit up bulb (5)
I believe the answer is:
tulip
'bulb' is the definition.
(I've seen this before)
'switch lit up' is the wordplay.
'switch' is an anagram indicator.
'lit'+'up'='litup'
'litup' with letters rearranged gives 'TULIP'.
(Other definitions for tulip that I've seen before include "Plant at the centre of the Dutch financial bubble of 1637" , "Spring-blooming plant" , "cause of mania once" , "Spring-flowering plant" , "Plant, it's name shares an origin with turban" .)