Collect a choral work (5)

I believe the answer is:

amass

I'm a little stuck... Click here to teach me more about this clue!

'collect' is the definition.
(I know that collect can be written as amass)

'a choral work' is the wordplay.
'choral work' becomes 'mass' (I can't explain this - if you can you should believe this answer much more).
'a'+'mass'='AMASS'

Can you help me to learn more?

(Other definitions for amass that I've seen before include "run up?" , "Gather in great quantity" , "Stockpile" , "Gathers" , "Assemble" .)

I've seen this clue in the Evening Standard.

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