Soldiers legally bound to be trustworthy (8)
I believe the answer is:
reliable
'be trustworthy' is the definition.
(associated in meaning)
'soldiers legally bound' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'legally bound' becomes 'liable' (liable for something).
're'+'liable'='RELIABLE'
'to' is the link.
(Other definitions for reliable that I've seen before include "Can be depended upon, trustworthy" , "Able to be depended on" , "worthy of trust" , "Trusty" , "Dependable, can be trusted" .)