Building that's essential to see? (9)
I believe the answer is:
cathedral
'building' is the definition.
(cathedral is a kind of building)
'that's essential to see?' is the wordplay.
'that' becomes 'cathedra' (I can't justify this - if you can you should give a lot more credence to this answer).
'essential to' says to take the centre.
'see?' becomes 'Ely' (see is a place with a cathedral - e.g. Ely).
The centre of 'ely' is 'l'.
'cathedra'+'l'='CATHEDRAL'
Can you help me to learn more?
(Other definitions for cathedral that I've seen before include "dramatic murder location" , "place of religion" , "See building" , "St Paul's or St Patrick's" , "religious building" .)