Soldiers trapped in circular object alien guarded (8)
I believe the answer is:
discreet
'guarded' is the definition.
(thesaurus)
'soldiers trapped in circular object alien' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'trapped in' is an insertion indicator.
'circular object' becomes 'disc' (I've seen this before).
'alien' becomes 'et' (extra terrestrial).
'disc'+'et'='discet'
're' inserted inside 'discet' is 'DISCREET'.
(Other definitions for discreet that I've seen before include "Careful not to attract attention" , "Cautious, doesn't give away secrets" , "Careful in not attracting attention" , "Careful to avoid embarrassment" , "Careful in keeping a secret" .)