Yours truly, extremely catty and hostile (3)
I believe the answer is:
icy
'hostile' is the definition.
(I've seen this before)
'yours truly extremely catty' is the wordplay.
'yours truly' becomes 'I' (the person who set this crossword).
'extremely' suggests removing the centre (only the extremes of the word are used).
'catty' with its centre removed is 'cy'.
'i'+'cy'='ICY'
'and' acts as a link.
(Other definitions for icy that I've seen before include "Frosty and slippy" , "unsympathetic" , "Very chilly" , "Very cold, unfriendly" , "Posing danger on the road" .)