Sibling rivalry's beginning? Worry about it! (7)
I believe the answer is:
brother
'sibling' is the definition.
(I've seen this before)
'rivalry's beginning? worry about it' is the wordplay.
'rivalry's beginning?' becomes 'r' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'worry' becomes 'bother' (I've seen this before).
'about it' means one lot of letters goes inside another.
'r' inserted inside 'bother' is 'BROTHER'.
Can you help me to learn more?
(Other definitions for brother that I've seen before include "Relation" , "Trade unionist" , "Fellow-member" , "Union member - male relative" , "Male sibling" .)