Relative in Barlinnie certainly (5)
I believe the answer is:
niece
'relative' is the definition.
(type of relative)
'in barlinnie certainly' is the wordplay.
'in' indicates the answer is hidden within the clue.
'NIECE' is hidden in the letters of 'barlinnie certainly'.
(Other definitions for niece that I've seen before include "A sibling's daughter" , "A younger female relative" , "A sibling's child" , "Son's cousin" , "Brother's girl" .)