Little brother wanted a cousin to show up (5)
I believe the answer is:
cadet
'little brother' is the definition.
(cadet could historically mean a younger sibling)
'wanted a cousin to show up' is the wordplay.
'to show' indicates the answer is hidden within the clue.
'up' is a reversal indicator (in down clue: letters go upwards).
'tedac' is hidden within 'wanted a cousin'.
'tedac' written backwards gives 'CADET'.
(Other definitions for cadet that I've seen before include "Junior officer; younger brother" , "officer of the future" , "Apprentice officer" , "aspiring officer" , "Younger son" .)