Let into circle, a senator (5)
I believe the answer is:
lease
'let' is the definition.
(as in to lease or let a building)
'into circle a senator' is the wordplay.
'into' says the answer is hidden in the clue.
'LEASE' is hidden within 'circle a senator'.
(Other definitions for lease that I've seen before include "Renting contract" , "Term of a rental contract" , "Contract of tenure" , "Summer's has too short a date (Shak.)" , "Contract allowing use of a property" .)