Girl’s top not returned, keeping her initially in bikini bottom? (5)
I believe the answer is:
thong
'bikini bottom?' is the definition.
'girl's top not returned keeping her initially' is the wordplay.
'top' indicates taking the first letters.
'returned' is a reversal indicator.
'keeping' indicates putting letters inside.
'initially' suggests taking the first letters.
The first letter of 'girl' is 'g'.
The first letter of 'her' is 'h'.
'g'+'not'='gnot'
'gnot' backwards is 'tong'.
'tong' placed around 'h' is 'THONG'.
'in' acts as a link.
(Other definitions for thong that I've seen before include "Pants" , "Insubstantial underwear" , "Narrow strip of leather - beach garment" , "Frequently (archaic)" , "Narrow strip of leather (or other material)" .)