Bird with only one little sister for relatives! (6)
I believe the answer is:
siskin
'bird with only one little' is the definition.
'siskin' can be an answer for 'bird' (I've seen this before). I am not sure about the 'with only one little' bit.
'sister for relatives' is the wordplay.
'sister' becomes 'sis' ('sis' can be a synonym of 'sister').
'for' is a charade indicator (letters next to each other).
'relatives' becomes 'kin' (I've seen this before).
'sis'+'kin'='SISKIN'
Can you help me to learn more?
(Other definitions for siskin that I've seen before include "Yellow-and-black finch" , "Yellow-green finch" , "Singer" , "Small songbird related to the goldfinch" , "Greenish songbird of finch family" .)