Holiday', a popular song penned by Welsh star (7)
I believe the answer is:
whitsun
'holiday'' is the definition.
(I know that Whitsun is a type of religious festival)
'a popular song penned by welsh star' is the wordplay.
'a popular song' becomes 'hit' (a hit song).
'penned by' is an insertion indicator (inserted letters are penned in).
'welsh' becomes 'W' (abbreviation).
'star' becomes 'sun' (the Sun is a star).
'w'+'sun'='wsun'
'hit' going within 'wsun' is 'WHITSUN'.
(Other definitions for whitsun that I've seen before include "Holy Spirit festival" , "Wins hut on short Christian Sabbath" , "Feast" , "Period seven weeks after Easter" , "Spring festival" .)