Crossword Genius

Doubtful character, notwithstanding that large amount of service in church (6)

Ross

I believe the answer is:

thomas

I'm a little stuck... Click here to teach me more about this clue!

'doubtful character notwithstanding that large' is the definition.
I can't judge whether this definition defines the answer.

'amount of service in church' is the wordplay.
'amount' becomes 'Th' (Th is an example).
'of' becomes 'o''.
'service in church' becomes 'mas' (I can't justify this - if you can you should believe this answer much more).
'th'+'o'+'mas'='THOMAS'

Can you help me to learn more?

(Other definitions for thomas that I've seen before include "fellow" , "Boy's name - sceptic" , "Recipient of 10 21 [VELODROME GOLD]" , "Famous tank engine" , "The doubtful Apostle" .)

I've seen this clue in The Independent.
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