A lax body -- most common in mums -- during surgery, finally tucked up? (14)

'a lax body most common in mums' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'during surgery finally tucked up?' is the wordplay.
I cannot quite see how this works, but
'during' could be 'in' (I've seen this in other clues) and 'in' is present in the answer.
'surgery' could be 'op' (short for operation) and 'op' is found in the answer.
'finally' could be 'last' (synonyms) and 'last' is present in the answer.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?