To ski erratically in extremes of rapidity is dangerous! (5)
I believe the answer is:
risky
'is dangerous' is the definition.
(similar in meaning)
'to ski erratically in extremes of rapidity' is the wordplay.
'erratically' is an anagram indicator.
'in' means one lot of letters goes inside another.
'extremes of' suggests removing the centre (only the letters at the extremes of the word).
'rapidity' with its centre removed is 'ry'.
'ski' anagrammed gives 'isk'.
'isk' placed within 'ry' is 'RISKY'.
(Other definitions for risky that I've seen before include "admitting possibility of unpleasant outcome" , "Perilous" , "too adventurous" , "Potentially dangerous" , "Hazardous" .)