Remove from roster a sentry (5)
I believe the answer is:
erase
'remove' is the definition.
(thesaurus)
'from roster a sentry' is the wordplay.
'from' says the answer is hidden in the clue.
'ERASE' is hidden within 'roster a sentry'.
(Other definitions for erase that I've seen before include "Eliminate" , "Blot out" , "score" , "Blank out" , "Put out of mind" .)