Cousin unexpectedly starts to appear less withdrawn (8)
I believe the answer is:
unsocial
'withdrawn' is the definition.
(similar in meaning)
'cousin unexpectedly starts to appear less' is the wordplay.
'unexpectedly' indicates an anagram.
'starts to' suggests taking the first letters.
The initial letters of 'appear less' is 'al'.
'cousin' anagrammed gives 'unsoci'.
'unsoci'+'al'='UNSOCIAL'
(Other definitions for unsocial that I've seen before include "not wanting to go with the crowd" , "These working hours inhibit the gregarious life" , "(Of working hours) falling outside the normal day" , "(Of work hours) outside the usual working day" .)