Most faithful perplexed about lock (8)

I believe the answer is:

loyalest

'most faithful' is the definition.
(loyal can mean faithful)

'perplexed about lock' is the wordplay.
'perplexed' becomes 'lost'.
'about' is an insertion indicator.
'lock' becomes 'yale' (I've seen this before).
'lost' placed around 'yale' is 'LOYALEST'.

I've seen this clue in The Telegraph.

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