Halliwell appears in rank underwear (8)
I believe the answer is:
lingerie
'underwear' is the definition.
(type of underwear)
'halliwell appears in rank' is the wordplay.
'halliwell' becomes 'geri' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'appears in' is an insertion indicator.
'rank' becomes 'line'.
'geri' going within 'line' is 'LINGERIE'.
Can you help me to learn more?
(Other definitions for lingerie that I've seen before include "Ladies' underwear" , "Slinky undergarments" , "Garments" , "frillies" , "pants! Could be" .)