Architrave from which to efface unknown letter (7)
I believe the answer is:
epistle
'letter' is the definition.
(I know that epistle is a type of letter)
'architrave from which to efface unknown' is the wordplay.
'architrave' becomes 'epistyle' (architectural term).
'from which to efface' suggests deleting specific letters.
'unknown' becomes 'y' (algebra).
'epistyle' with 'y' taken out is 'EPISTLE'.
(Other definitions for epistle that I've seen before include "may be written off" , "Apostle's letter" , "A letter, biblical perhaps" , "Pets lie (anag)" , "Romans perhaps" .)