... could be enigma, I suppose (7)
I believe the answer is:
imagine
'suppose' is the definition.
(I know that suppose can be written as imagine)
'could be enigma i' is the wordplay.
'could be' is an anagram indicator.
'enigma'+'i'='enigmai'
'enigmai' with letters rearranged gives 'IMAGINE'.
(Other definitions for imagine that I've seen before include "Form a mental picture" , "A Gemini? (anag)" , "See something that doesn't exist" , "Picture to oneself" , "John Lennon classic (just think of it!)" .)