Think I designed enigma? (7)
I believe the answer is:
imagine
'think' is the definition.
(I know that think can be written as imagine)
'i designed enigma?' is the wordplay.
'designed' is an anagram indicator (designed with the letters).
'enigma' anagrammed gives 'magine'.
'i'+'magine'='IMAGINE'
(Other definitions for imagine that I've seen before include "Reckon" , "John Lennon hit" , "Picture, conceive" , "that Lennon composed" , "Suppose; conceive" .)