Extremely restive young relative without a cause (6)
I believe the answer is:
reason
'cause' is the definition.
(I know that cause can be written as reason)
'extremely restive young relative without a' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'young relative' becomes 'son' (a child).
'without' means one lot of letters goes inside another ('without' can be similar in meaning to 'outside').
'restive' with its middle removed is 're'.
're'+'son'='reson'
'reson' placed around 'a' is 'REASON'.
(Other definitions for reason that I've seen before include "Motivation or cause" , "Power of thought" , "Explanation for some event" , "Rational motive" , "Cause to perform" .)