Relate to an apparently certain evacuation (9)

I believe the answer is:

appertain

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'relate' is the definition.
(I've seen this before)

'to an apparently certain evacuation' is the wordplay.
'to an apparently' becomes 'app' (I've seen this in other clues).
'certain evacuation' becomes 'ertain' (I can't explain this - if you can you should give a lot more credence to this answer).
'app'+'ertain'='APPERTAIN'

Can you help me to learn more?

(Other definitions for appertain that I've seen before include "Belong, relate (to)" , "Bear on" , "Concern" , "Be relevant to" .)

I've seen this clue in the Sydney Morning Herald.

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