Design it? I once had concealed lighting (8)
I believe the answer is:
ignition
'lighting' is the definition.
('ignition' can be a synonym of 'lighting')
'design it? i once had concealed' is the wordplay.
'concealed' indicates a hidden word.
'IGNITION' can be found hidden inside 'design it i once had'.
(Other definitions for ignition that I've seen before include "Car engine starter" , "Process of starting car" , "Mechanism firing up an internal-combustion engine" , "Act of starting a fire" , "Mechanism used to start an engine" .)