Tear off at speed (4)
I believe the answer is:
rate
'speed' is the definition.
(the speed at which something happens)
'tear off' is the wordplay.
'off' is an anagram indicator.
'tear' anagrammed gives 'RATE'.
'at' acts as a link.
(Other definitions for rate that I've seen before include "Be worthy of; speed" , "Charge per unit" , "Relative speed (of progress)" , "Adjudge" , "Capital of the Philippines" .)