Spiral into the extremes of ghastly speed (6)
I believe the answer is:
gyrate
'spiral' is the definition.
(I know that spiral can be written as gyrate)
'the extremes of ghastly speed' is the wordplay.
'the extremes of' suggests removing the centre (only the letters at the extremes of the word).
'speed' becomes 'rate' (the speed at which something happens).
'ghastly' with its middle taken out is 'gy'.
'gy'+'rate'='GYRATE'
'into' acts as a link.
(Other definitions for gyrate that I've seen before include "Spin at speed" , "Dance wildly" , "Turn" , "Spin round" , "Move in a spiral course" .)