Partly regrets isolating flipping relative (6)
I believe the answer is:
sister
'relative' is the definition.
(I know that Sister is a type of sibling)
'partly regrets isolating flipping' is the wordplay.
'partly' indicates the answer is hidden within the clue (part of the clue is used).
'flipping' shows that the letters should be reversed in order.
'retsis' can be found hidden inside 'regrets isolating'.
'retsis' written backwards gives 'SISTER'.
(Other definitions for sister that I've seen before include "Member of family" , "Senior nurse; relative" , "Relative in hospital?" , "Female sibling" , "The nurse in charge" .)