Astonish someone who has forgotten it is now April? (6,1,8)

I believe the answer is:

spring a surprise

'astonish' is the definition.
(thesaurus)

'someone who has forgotten it is now april?' is the wordplay.
I cannot quite understand how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is present in the answer.
'is' is present in the answer.
This explanation may well be incorrect...

I've seen this clue in The Guardian.

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