"Think of it . . ." (7)
'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" .)