While being faithful, deceive a saint with a testament (8)

I believe the answer is:

constant

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'while being faithful' is the definition.
I can't tell whether this defines the answer.

'deceive a saint with a testament' is the wordplay.
'deceive' becomes 'con' (I've seen this before).
'a saint' becomes 'st'.
'with' says to put letters next to each other.
'testament' becomes 'nt'.
'con'+'st'+'a'+'nt'='CONSTANT'

Can you help me to learn more?

(Other definitions for constant that I've seen before include "Steadfast or unvarying" , "Resolute" , "without changes" , "Continuing unvarying" , "Continuous" .)

I've seen this clue in the Evening Standard.

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