Fifteen planned to take on 101, working with speed and economy (9)
I believe the answer is:
efficient
'economy' is the definition.
(I know that economy can be written as efficient)
'fifteen planned to take on 101' is the wordplay.
'planned' is an anagram indicator.
'to take on' indicates putting letters inside.
'101' becomes 'CI' (roman numerals).
'fifteen' is an anagram of 'effient'.
'effient' placed around 'ci' is 'EFFICIENT'.
'working with speed and' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for efficient that I've seen before include "Not wasting energy" , "Performing with little wastage" , "Functioning effectively" , "Effective in doing what is required" , "Businesslike" .)