Turn it up to secure starter for light bulb (5)
I believe the answer is:
tulip
'bulb' is the definition.
(I've seen this before)
'turn it up to secure starter for light' is the wordplay.
'turn' indicates an anagram.
'to secure' means one lot of letters goes inside another (inserted letters are secured inside).
'starter for' says to take the initial letters.
The initial letter of 'light' is 'l'.
'it'+'up'='itup'
'itup' is an anagram of 'tuip'.
'tuip' enclosing 'l' is 'TULIP'.
(Other definitions for tulip that I've seen before include "Amsterdam bloom?" , ""Dutch" flower" , "cause of mania once" , "Spring-blooming plant" , "Spring-flowering bulb" .)