Extreme danger outside since nervous reaction (7)
I believe the answer is:
drastic
'extreme' is the definition.
(thesaurus)
'danger outside since nervous reaction' is the wordplay.
'outside' suggests removing the centre.
'since' becomes 'as' (I've seen this in other clues).
'nervous reaction' becomes 'tic' (I've seen this in other clues).
'danger' with its middle removed is 'dr'.
'dr'+'as'+'tic'='DRASTIC'
(Other definitions for drastic that I've seen before include "Thoroughgoing" , "Extreme - forceful" , "Extreme - violent" , "Extremely severe" , "Radical, extreme" .)