Fastest agreement that is initially signed in (7)

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

'agreement that is initially signed in' is the wordplay.
'agreement' becomes 'pact' (a pact is an agreement or deal).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'initially' indicates taking the first letters.
'in' indicates putting letters inside.
The initial letter of 'signed' is 's'.
'ie'+'s'='ies'
'pact' enclosing 'ies' is 'PACIEST'.

Can you help me to learn more?