Bones that are rarely seen in heads (5)
I believe the answer is:
tarsi
'bones' is the definition.
(I've seen this before)
'that are rarely seen in heads' is the wordplay.
'heads' suggests taking the first letters.
The first letters of 'that are rarely seen in' is 'TARSI'.
(Other definitions for tarsi that I've seen before include "parts of leg" , "Ankle bones" , "Foot bones" , "Bones of the ankles and heels" , "Parts of skeleton" .)