Thanks young relative restraining extremely rare disloyalty (7)
I believe the answer is:
treason
'disloyalty' is the definition.
(I know that treason is a type of disloyalty)
'thanks young relative restraining extremely rare' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'young relative' becomes 'son' (a child).
'restraining' means one lot of letters goes inside another.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'rare' with its centre taken out is 're'.
'ta'+'son'='tason'
'tason' placed around 're' is 'TREASON'.
(Other definitions for treason that I've seen before include "Stare on (anag.)" , "Mutiny" , "Serious crime" , "Offence against state" , "Act of betrayal of a government" .)