Work out volume — something under a foot all round? (5)

I believe the answer is:

solve

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'work out' is the definition.
(I know that work out can be written as solve)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for solve that I've seen before include "Find the key - work out" , "Loves (anag) - to find an answer" , "Find an answer to (a problem)" , "Get the answers" , "Work out (a clue)" .)

I've seen this clue in The Times.

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