Part of shirt buttonhole (6)
I believe the answer is:
collar
I believe this clue is a double definition.
'part of shirt' is the first definition.
(I've seen this before)
'buttonhole' is the second definition.
(both can mean to detain)
(Other definitions for collar that I've seen before include "Neckband to seize" , "One with "dog" identifies a clergyman" , "A cloth neck-band" , "Nick" , "by the lapels?" .)