A little fossil I acquired forming part of pelvis (5)
I believe the answer is:
iliac
'part of pelvis' is the definition.
Although both the answer and definition are adjectives, I can't see how they can define each other.
'a little fossil i acquired' is the wordplay.
'a little' indicates a hidden word.
'ILIAC' can be found hidden inside 'fossil i acquired'.
'forming' acts as a link.
Can you help me to learn more?
(Other definitions for iliac that I've seen before include "Relating to part of the hip bone" , "Trojan" , "Pertaining to hip bone" , "bottom of a hip bone" , "Like certain body parts" .)