Theologian going in to feed is upset, seeing cultural gathering (10)
I believe the answer is:
eisteddfod
'cultural gathering' is the definition.
(type of Welsh cultural gathering)
'theologian going in to feed is upset' is the wordplay.
'theologian' becomes 'dd' (Doctor of Divinity degree).
'going in' means one lot of letters goes inside another.
'upset' indicates an anagram.
'to'+'feed'+'is'='tofeedis'
'tofeedis' anagrammed gives 'eistefod'.
'dd' placed within 'eistefod' is 'EISTEDDFOD'.
'seeing' acts as a link.
(Other definitions for eisteddfod that I've seen before include "Welsh cultural festival" , "arts competition" , "Celtic festival" , "Welsh arts festival" , "Welsh art festival" .)