Mocking son, very cold about a sister at first (9)
I believe the answer is:
sarcastic
'mocking' is the definition.
(I've seen this before)
'son very cold about a sister at first' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'very cold' becomes 'arctic' (with arctic temperatures).
'about' means one lot of letters goes inside another.
'at first' suggests taking the first letters.
The initial letter of 'sister' is 's'.
'a'+'s'='as'
'arctic' enclosing 'as' is 'arcastic'.
's'+'arcastic'='SARCASTIC'
(Other definitions for sarcastic that I've seen before include "Pungent" , "Using mocking wit" , "Ironically mocking" , "Cruelly ironical" , "Using mocking irony" .)