Parent, briefly worried supporting favourite pantomime (5,3)
I believe the answer is:
peter pan
'pantomime' is the definition.
(I've seen this before)
'parent briefly worried supporting favourite' is the wordplay.
'briefly' means to remove the last letter.
'worried' indicates an anagram.
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'favourite' becomes 'pet' (synonyms).
'parent' with its last letter taken away is 'paren'.
'paren' with letters rearranged gives 'erpan'.
'erpan' after 'pet' is 'PETER PAN'.
(Other definitions for peter pan that I've seen before include "JM Barrie character" , "Ever-young person" , "Ever-youthful boy" , "Child that never grows old" , "J M Barrie's immortal boy" .)