Henna used by church to embellish (7)
I believe the answer is:
enhance
'embellish' is the definition.
(thesaurus)
'henna used by church' is the wordplay.
'used' is an anagram indicator.
'by' is a charade indicator (letters next to each other).
'church' becomes 'CE' (abbreviation for Church of England).
'henna' anagrammed gives 'enhan'.
'enhan'+'ce'='ENHANCE'
'to' is the link.
(Other definitions for enhance that I've seen before include "Improve, make more attractive" , "Improve the quality of (something)" , "Improve quality (of item)" , "Increase attractiveness of, improve" , "Make better or more attractive" .)