Quick to learn and understand (6)
I believe the answer is:
clever
'quick to learn and understand' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for clever that I've seen before include "Quick at learning, talented" , "Sharp-witted, intelligent" , "Smar and canny" , "Deft, able, bright" , "Highly intelligent" .)