Deny soldiers about to intercept aircraft (6)
I believe the answer is:
reject
'deny' is the definition.
(thesaurus)
'soldiers about to intercept aircraft' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'about' becomes 'c' (abbreviation for 'circa').
'to intercept' means one lot of letters goes inside another.
'aircraft' becomes 'jet' (jet is a kind of aircraft).
'c' placed into 'jet' is 'ject'.
're'+'ject'='REJECT'
(Other definitions for reject that I've seen before include "Refuse to accept, send back" , "Refuse to accept, repulse" , "Decline" , "Dismiss (as faulty)" , "Refuse as inadequate or unworthy" .)