Tough soldiers tight-lipped about start of invasion (9)
I believe the answer is:
resilient
'tough' is the definition.
(I've seen this before)
'soldiers tight-lipped about start of invasion' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'tight-lipped' becomes 'silent'.
'about' is an insertion indicator.
'start of' says to take the initial letters.
The first letter of 'invasion' is 'i'.
'silent' enclosing 'i' is 'silient'.
're'+'silient'='RESILIENT'
(Other definitions for resilient that I've seen before include "Hardy" , "able to withstand disappointment" , "bouncing back" , "Bouncy" , "Rebounding from impact" .)