Crossword Genius

Be quick, John — knee’s beginning to hurt (4,5)

Ross

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" .)

I've seen this clue in The Hindu.
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