Relative in detention centre an Indian copper once (5)
I believe the answer is:
nanna
'relative' is the definition.
(I know that nanna is a type of relative)
'detention centre an indian copper once' is the wordplay.
'centre' indicates the central letters.
'an indian copper once' becomes 'anna' (historical coin of India).
The centre of 'detention' is 'n'.
'n'+'anna'='NANNA'
'in' is the link.
(Other definitions for nanna that I've seen before include "Grandma" , "Senior woman" , "Child's grandmother" , "Gran" , "Mother's mother?" .)