Relative flipped codpiece into trousers (5)
I believe the answer is:
niece
'relative' is the definition.
(type of relative)
'flipped codpiece into trousers' is the wordplay.
'flipped' says the letters should be written in reverse.
'trousers' indicates a hidden word (trouser can mean to steal or take).
'ecein' can be found hidden inside 'codpiece into'.
'ecein' written backwards gives 'NIECE'.
(Other definitions for niece that I've seen before include "Brother's daughter" , "A younger female relative" , "Brother or sister's daughter" , "Family relative" , "A sibling's child" .)