Relatives terribly regal in play (4,4)
I believe the answer is:
king lear
'play' is the definition.
(I've seen this before)
'relatives terribly regal in' is the wordplay.
'relatives' becomes 'kin' (I've seen this before**).
'terribly' is an anagram indicator.
'in' is an insertion indicator.
'regal' with letters rearranged gives 'glear'.
'kin' going within 'glear' is 'KING LEAR'.
(Other definitions for king lear that I've seen before include "Tragic Shakespearean monarch" , "One of Shakespeare's tragedies" , "Tragic Shakespeare monarch" , "Tragic hero" , "Shakespearian royal tragedy" .)