A case of hefty unit oddly not used (4)
I believe the answer is:
etui
'a case' is the definition.
(case for needlework equipment)
'hefty unit oddly not used' is the wordplay.
'oddly not used' indicates alternate letters (letters in odd-numbered positions removed).
'hefty'+'unit'='heftyunit'
The alternate letters of 'heftyunit' are 'ETUI'.
'of' is the link.
(Other definitions for etui that I've seen before include "Ornamental ladies' bag for small articles" , "Needlework case" , "Small sewing bag" , "Small case for small things" , "Case for holding sewing articles" .)