Should this not be 'a man'? (4)

I believe the answer is:
amen
'should this not' is the definition.
The definition suggests a singular noun which matches the answer.
''a man'?' is the wordplay.
'man'?' becomes 'men' (I am not sure about this - if you are sure you should believe this answer much more).
'a'+'men'='AMEN'
'be' acts as a link.
Can you help me to learn more?
(Other definitions for amen that I've seen before include "Name for end to prayer" , "Final word in prayer" , "I agree!" , "prayer's ending" , "Conclusion of prayer" .)
