Last word before Spanish aunt shows severe congenital mental deficiency (7)

'mental deficiency' is the definition.
(amentia is a kind of mental deficiency)

'last word before spanish aunt' is the wordplay.
'last word' becomes 'amen' (last word in a prayer).
'before' means one lot of letters go next to another.
'spanish aunt' becomes 'tia' ('aunt' in Spanish).
'amen'+'tia'='AMENTIA'

'shows severe congenital' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.

Can you help me to learn more?