As eldest child, male, I severely criticise Dad (4,8)
I believe the answer is:
heir apparent
'as eldest child' is the definition.
(an heir apparent might be someone's eldest child)
'male i severely criticise dad' is the wordplay.
'male' becomes 'he' (he refers to a man).
'severely criticise' becomes 'rap' (rap can mean to criticise).
'dad' becomes 'parent'.
'he'+'i'+'rap'+'parent'='HEIR-APPARENT'
(Other definitions for heir apparent that I've seen before include "successor eventually" , "Acknowledged beneficiary" , "Next in line to throne" , "sure of inheritance" , "One expected to succeed" .)