Allowed astronomer to leave Earth, with fatal consequences (8)
I believe the answer is:
lethally
'with fatal consequences' is the definition.
'allowed astronomer to leave earth' is the wordplay.
'allowed' becomes 'let' (I've seen this before).
'astronomer' becomes 'halley' (Edmond Halley).
'to leave' suggests deleting specific letters.
'earth' becomes 'e'.
'halley' with 'e' taken out is 'hally'.
'let'+'hally'='LETHALLY'
(Other definitions for lethally that I've seen before include "in fatal fashion" , "with deadly consequences" .)