The first to twitch curtain is weirdly antisocial (8)
I believe the answer is:
taciturn
'antisocial' is the definition.
(both can mean untalkative or withdrawn)
'the first to twitch curtain is weirdly' is the wordplay.
'the first to' says to take the initial letters.
'is weirdly' indicates anagramming the letters.
The initial letter of 'twitch' is 't'.
'curtain' anagrammed gives 'aciturn'.
't'+'aciturn'='TACITURN'
(Other definitions for taciturn that I've seen before include "Tending to silence" , "Reserved, saying little" , "Untalkative" , "Without saying much" , "Disinclined to speak" .)