Parent mostly sad after favourite play (5,3)
I believe the answer is:
peter pan
'play' is the definition.
(I've seen this before)
'parent mostly sad after favourite' is the wordplay.
'mostly' means to remove the last letter.
'sad' indicates anagramming the letters.
'after' says to put letters next to each other.
'favourite' becomes 'pet' ('pet' can be a synonym of 'favourite').
'parent' with its last letter taken away is 'paren'.
'paren' with letters rearranged gives 'erpan'.
'erpan' put after 'pet' is 'PETER PAN'.
(Other definitions for peter pan that I've seen before include "Boyish creation of J.M. Barrie" , "Kind of collar for" , "Ever-young boy of James Barrie's play" , "voyager to Never Never Land" , "Type of eternal youth" .)