Good length of race not beginning in slow style (7)
I believe the answer is:
glacial
'in slow style' is the definition.
'good length of race not beginning' is the wordplay.
'good' becomes 'g' (abbreviation).
'length' becomes 'l'.
'of race' becomes 'racial'.
'not beginning' means to remove the first letter.
'racial' with its first letter removed is 'acial'.
'g'+'l'+'acial'='GLACIAL'
(Other definitions for glacial that I've seen before include "Pertaining to the ice ages" , "Freezing - unfriendly" , "Frigid" , "Ice cold" , "Icy; distant" .)