The most common and most likely answer to this crossword clue is the 7 letter word DIPTYCH
I've seen this crossword clue in The Guardian
D
I
P
T
Y
C
H
Buddy explains!
The straight part
We know that "altarpiece" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case altarpiece = diptych.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DIPTYCH mean?
DIPTYCH noun- A writing tablet consisting of two leaves of rigid material connected by hinges and shutting together so as to protect the writing within.
- A picture or series of pictures painted on two tablets, usually connected by hinges.
- A double catalogue, containing in one part the names of living, and in the other of deceased, ecclesiastics and benefactors of the church.
- A catalogue of saints.
- Artistically-wrought tablets distributed by consuls, etc. of the later Roman Empire to commemorate their tenure of office; hence transferred to a list of magistrates