Knock Pat up (3)

I believe the answer is:

tap

'knock' is the definition.
(I know that knock can be written as tap)

'pat up' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'pat' in reverse letter order is 'TAP'.

(Other definitions for tap that I've seen before include "Knock gently" , "Somewhere to drink beer" , "Strike lightly" , "Make use of; water from it" , "Stopper enabling withdrawal of liquid" .)

I've seen this clue in the Evening Standard.

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