English divine visiting new site of key Welsh festival (10)
I believe the answer is:
eisteddfod
'welsh festival' is the definition.
(Welsh cultural festival)
'english divine visiting new site of key' is the wordplay.
'english' becomes 'e' (abbreviation).
'divine' becomes 'dd' (divine can mean theologian eg Doctor of Divinity).
'visiting' means one lot of letters goes inside another.
'new' is an anagram indicator.
'key' becomes 'd' (musical key).
'site'+'of'='siteof'
'siteof' with letters rearranged gives 'istefo'.
'dd' put within 'istefo' is 'isteddfo'.
'e'+'isteddfo'+'d'='EISTEDDFOD'
(Other definitions for eisteddfod that I've seen before include "Welsh art festival" , "Welsh cultural festival" , "arts competition" , "Welsh arts festival" , "Celtic festival" .)