Ingenuity shown by relative, it's said, in festival (7)
I believe the answer is:
whitsun
'festival' is the definition.
(Christian holiday)
'ingenuity shown by relative it's said' is the wordplay.
'ingenuity' becomes 'whit' (I can't justify this - if you can you should believe this answer much more).
'shown by' is a charade indicator (letters next to each other).
'relative' becomes 'son' (son is a kind of relative).
'it's said' indicates a 'sounds like' (homophone) clue.
'son' is a homophone of 'sun'.
'whit'+'sun'='WHITSUN'
'in' acts as a link.
(Other definitions for whitsun that I've seen before include "Spring festival" , "Holy Spirit festival" , "Wins hut on short Christian Sabbath" , "holiday?" , "Feast" .)