Prepared to run whenever required (2,3)
I believe the answer is:
on tap
'prepared' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for on tap that I've seen before include "Immediately available like beer in pub" , "Ready to be drawn off, like beer" , "Freely available" , "readily accessible" , "Constantly available like beer from keg" .)