Capable Charlie means to apply pressure (6)
I believe the answer is:
clever
'capable' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for clever that I've seen before include "easy to teach?" , "brilliant" , "Quick-witted, smart" , "Smart or shrewd" , "Sharp-witted, intelligent" .)