Allow a thousand to enter passage (5)
I believe the answer is:
admit
'allow' is the definition.
(admit can mean to allow or accept)
'a thousand to enter passage' is the wordplay.
'a thousand' becomes 'M' (Roman numeral for a thousand).
'to enter' means one lot of letters goes inside another.
'passage' becomes 'adit' (passage leading to a mine).
'm' inserted within 'adit' is 'ADMIT'.
(Other definitions for admit that I've seen before include "Take on" , "Allow entry" , "Allow to enter; own up" , "Come clean" , "Agree to the truth" .)