Soldiers quietly crossing island with flexibility (11)
I believe the answer is:
resiliently
'soldiers quietly crossing island with flexibility' is the definition.
I can't tell whether this definition defines the answer.
'soldiers quietly crossing island with' is the wordplay.
'soldiers' becomes 'RE' (Royal Engineers).
'quietly' becomes 'silently' (I've seen this before).
'crossing' indicates putting letters inside.
'island with' becomes 'i' (I can't justify this - if you can you should give a lot more credence to this answer).
'silently' placed around 'i' is 'siliently'.
're'+'siliently'='RESILIENTLY'
Can you help me to learn more?
(Another definition for resiliently that I've seen is " How to cope with disappointment".)