Adolescent showing support with flipping king's daughter (8)
I believe the answer is:
teenager
'adolescent' is the definition.
(I know that adolescent can be written as teenager)
'support with flipping king's daughter' is the wordplay.
'support' becomes 'tee' (tee is a kind of support).
'with' says to put letters next to each other.
'flipping' is a reversal indicator.
'king's daughter' becomes 'Regan' (in Shakespeare's King Lear).
'regan' back-to-front is 'nager'.
'tee'+'nager'='TEENAGER'
'showing' acts as a link.
(Other definitions for teenager that I've seen before include "one no longer twelve?" , "14, perhaps" , "Old child or young adult?" , "Youngster" , "hormonal problem?" .)