False Messiah, nut showing zeal (10)
I believe the answer is:
enthusiasm
'zeal' is the definition.
(I've seen this before)
'false messiah nut' is the wordplay.
'false' is an anagram indicator.
'messiah'+'nut'='messiahnut'
'messiahnut' anagrammed gives 'ENTHUSIASM'.
'showing' is the link.
(Other definitions for enthusiasm that I've seen before include "spirit?" , "Avidity" , "Zeal" , "Gusto; fervour" , "Intense and lively interest" .)