Refuse to admit about a hundred in plane (6)
I believe the answer is:
reject
'refuse to admit' is the definition.
(thesaurus)
'about a hundred in plane' is the wordplay.
'about' becomes 're' (regarding**).
'a hundred' becomes 'c' (roman numeral).
'in' is an insertion indicator.
'plane' becomes 'jet' (jet plane is a kind of plane).
'c' going into 'jet' is 'ject'.
're'+'ject'='REJECT'
(Other definitions for reject that I've seen before include "Refuse to consider (something)" , "Refuse to accept, send back" , "Refuse to accept or react against" , "Refuse to accept or acknowledge" , "Refuse to accept, spurn" .)