No huge complex is adequate (6)
I believe the answer is:
enough
'adequate' is the definition.
('enough' can be a synonym of 'adequate')
'no huge complex' is the wordplay.
'complex' is an anagram indicator.
'no'+'huge'='nohuge'
'nohuge' anagrammed gives 'ENOUGH'.
'is' acts as a link.
(Other definitions for enough that I've seen before include "As much as necessary" , "To the required extent" , "` `A word to the wise is ......' '" , "Adequate, sufficient" , "satisfying" .)