Geraldine appears in rank undergarments (8)
I believe the answer is:
lingerie
'undergarments' is the definition.
(I have seen 'Slinky undergarments' mean 'lingerie' so perhaps 'undergarments' could also mean 'lingerie')
'geraldine appears in rank' is the wordplay.
'geraldine' becomes 'geri' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'appears in' means one lot of letters goes inside another.
'rank' becomes 'line'.
'geri' put within 'line' is 'LINGERIE'.
Can you help me to learn more?
(Other definitions for lingerie that I've seen before include "flimsy stuff" , "Women's underclothes" , "pants! Could be" , "Women's underwear and nightclothes" , "Female underwear" .)