Polite beginner is likely to be flexible (7)
I believe the answer is:
pliable
'flexible' is the definition.
(I know that flexible can be written as pliable)
'polite beginner is likely' is the wordplay.
'beginner' says to take the initial letters.
'is likely' becomes 'liable' (similar in meaning).
The first letter of 'polite' is 'p'.
'p'+'liable'='PLIABLE'
'to be' acts as a link.
(Other definitions for pliable that I've seen before include "Easily bent" , "Elastic" , "Flexible" , "accommodating" , "Ductile" .)