Think of it! (4)
'think of it' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for idea that I've seen before include "Fancy" , "Mental impression" , "Light bulb above head" , "Thought, conception" , "Brainwave" .)
(Other definitions for imagine that I've seen before include "A Gemini? (anag)" , "John Lennon hit" , "Picture in one's mind" , "Form a mental image in the mind" , "that Lennon composed" .)