One enigma, terribly fancy (7)
I believe the answer is:
imagine
'fancy' is the definition.
(thesaurus)
'one enigma terribly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'terribly' is an anagram indicator.
'enigma' with letters rearranged gives 'magine'.
'i'+'magine'='IMAGINE'
(Other definitions for imagine that I've seen before include "Picture in mind, envisage" , "Reckon" , "Conjure up in mind,fancy" , "Assume" , "1971 John Lennon song" .)