Adolescent themes regularly annoy the Queen (8)
I believe the answer is:
teenager
'adolescent' is the definition.
(synonyms)
'themes regularly annoy the queen' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'annoy' becomes 'nag' (nag can mean to complain annoyingly).
'the queen' becomes 'ER' (abbreviation for Elizabeth Regina).
The alternate letters of 'themes' are 'tee'.
'tee'+'nag'+'er'='TEENAGER'
(Other definitions for teenager that I've seen before include "one with an unpredictable nature" , "Adolescent" , "one no longer twelve?" , "troublesome youth" , "14, perhaps" .)