Sum up posture: worryingly cocky (12)
I believe the answer is:
presumptuous
'cocky' is the definition.
(both can mean arrogant)
'sum up posture worryingly' is the wordplay.
'worryingly' is an anagram indicator (letters are disturbed or worried).
'sum'+'up'+'posture'='sumupposture'
'sumupposture' anagrammed gives 'PRESUMPTUOUS'.
(Other definitions for presumptuous that I've seen before include "Impertinently bold" , "Assuming too much" , "Impertinent" , "Bold - forward" .)