Be quick, John — knee’s beginning to hurt (4,5)
I believe the answer is:
look smart
'be quick' is the definition.
(I've seen this before)
'john knee's beginning to hurt' is the wordplay.
'john' becomes 'loo' (loo is a kind of john).
'beginning to' suggests taking the first letters.
'hurt' becomes 'smart' (synonyms).
The first letter of 'knee' is 'k'.
'loo'+'k'+'smart'='LOOK SMART'
(Other definitions for look smart that I've seen before include "Be quick!" , "appear fashionable and upmarket" , "Act quickly" .)