Twisting in roots is unusual (7)
I believe the answer is:
torsion
'twisting' is the definition.
(thesaurus)
'in roots is unusual' is the wordplay.
'is unusual' indicates an anagram.
'in'+'roots'='inroots'
'inroots' with letters rearranged gives 'TORSION'.
(Other definitions for torsion that I've seen before include "twist" , "The act of twisting" , "White-berried parasite" , "Tortuous and twisted shape or position" , "Process of twisting" .)