Pretend the pound note is coming back (3,2)
I believe the answer is:
let on
'pretend' is the definition.
(I've seen this before)
'the pound note is coming back' is the wordplay.
'the pound' becomes 'l'.
'is coming back' says the letters should be written backwards.
'note' written backwards gives 'eton'.
'l'+'eton'='LET ON'
(Other definitions for let on that I've seen before include "Reveal, divulge" , "Allow to board, or pretend" , "Tell me" , "Pretend informally or permit to board" , "Divulge info; pretend" .)