Extremely snappy single speed (5)
I believe the answer is:
irate
'extremely snappy' is the definition.
(irate can mean snappy or cross)
'single speed' is the wordplay.
'single' becomes 'I' (one).
'speed' becomes 'rate' (the speed at which something happens).
'i'+'rate'='IRATE'
(Other definitions for irate that I've seen before include "Annoyed, nettled" , "Angry, annoyed" , "Extremely cross" , "become irritated" , "Angry - furious" .)