Spur is sharp (6)
I believe the answer is:
prompt
This clue is a double definition.
'spur' is the first definition.
(thesaurus)
'sharp' is the second definition.
(thesaurus)
'is' acts as a link.
(Other definitions for prompt that I've seen before include "Immediate - supply forgotten words" , "Immediate - supply actor's lines" , "effective in expedition" , "Ready" , "Punctual; instigate" .)