A relative in hospital (6)

I believe the answer is:

sister

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'a relative in hospital' is the definition.
(I've seen this before)

'a relative in hospital' is the wordplay.
'a relative' becomes 'sis' (short for sister).
'in hospital' becomes 'ter' (I can't justify this - if you can you should give a lot more credence to this answer).
'sis'+'ter'='SISTER'

Can you help me to learn more?

(Other definitions for sister that I've seen before include "'Nurse, nun or relative (6)'" , "habits' - one who has them" , "Resits (anag.)" , "The nurse in charge" , "Relative in hospital?" .)

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