Keen to be prompt (5)
I believe the answer is:
ready
This clue is a double definition.
'keen' is the first definition.
(thesaurus)
'be prompt' is the second definition.
(similar in meaning)
'to' acts as a link.
(Other definitions for ready that I've seen before include "Quick" , "Prepare < dough" , "Waiting" , "All set to go" , "Prepared, willing" .)