Surprise day out in Streatham (5)

I believe the answer is:

treat

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'surprise day out' is the definition.
'treat' can be an answer for 'surprise' (I've seen this before). I'm not certain of the 'day out' bit.

'in streatham' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TREAT' is hidden in the letters of 'streatham'.

Can you help me to learn more?

(Other definitions for treat that I've seen before include "Deal with - delightful surprise" , "Something that causes special pleasure or delight" , "Tetra (anag.)" , "Cure" , "Try to heal" .)

I've seen this clue in The Sun.

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