Toupees needed for the principal parts (10)
I believe the answer is:
headpieces
'toupees' is the definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Perhaps you can see an association between them that I can't see?
'the principal parts' is the wordplay.
'the principal' becomes 'head' ('head' can be a synonym of 'principal').
'parts' becomes 'pieces' ('piece' can be a synonym of 'part').
'head'+'pieces'='HEADPIECES'
'needed for' is the link.
Can you help me to learn more?
(Another definition for headpieces that I've seen is " What may you see nuts with".)