Those detained in a series of engagements? (9,2,3)

'those detained' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'a series of engagements?' is the wordplay.
I cannot quite see how this works, but
'a' could be 'one' and 'one' is present in the answer.
'series' could be 'war' (I have seen 'series of engagements' mean 'war' so perhaps 'series' could also mean 'war') and 'war' is found within the answer.
'of' could be 'o' and 'o' is located in the answer.
This explanation may well be incorrect...

'in' acts as a link.

Can you help me to learn more?