Back choice between daughter and tot (6)

I believe the answer is:

dorsum

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'back' is the definition.
(I know that back can be written as dorsum)

'choice between daughter and tot' is the wordplay.
'choice' becomes 'or' (I can't explain this - if you can you should believe this answer much more).
'between' indicates putting letters inside.
'daughter' becomes 'd' (genealogical abbreviation).
'and' means one lot of letters go next to another.
'tot' becomes 'sum' (synonyms).
'd'+'sum'='dsum'
'or' put into 'dsum' is 'DORSUM'.

Can you help me to learn more?

I've seen this clue in The Times.

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