Area right between west end of nave and entrance (7)

'area' is the definition.
(narthex is a kind of area)

'right between west end of nave and entrance' is the wordplay.
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'between' means one lot of letters goes inside another.
'west end of' indicates taking the first letters (the leftmost letter of the word).
'entrance' becomes 'hex' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The initial letters of 'nave and' is 'na'.
'na'+'hex'='nahex'
'rt' going into 'nahex' is 'NARTHEX'.

Can you help me to learn more?