KGB, say, saving Philby time, which is most revealing (9)

'most revealing' is the definition.
Both the definition and answer are superlative adjectives. Maybe they are linked in a way I don't understand?

'kgb say saving philby time' is the wordplay.
I cannot quite understand how this works, but
'philby' could be 'kim' (spy Kim Philby) and 'kim' is present in the answer.
'time' could be 'est' (EST is an example) and 'est' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

'which is' acts as a link.

Can you help me to learn more?