Soldiers apt to be trustworthy (8)
I believe the answer is:
reliable
'be trustworthy' is the definition.
('reliable' is associated in meaning with 'trustworthy')
'soldiers apt' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'apt' becomes 'liable' ('liable' can be a synonym of 'apt'**).
're'+'liable'='RELIABLE'
'to' is the link.
(Other definitions for reliable that I've seen before include "Trusty" , "Dependable, trustworthy" , "Dependable, can be trusted" , "Can be depended upon, trustworthy" , "Unfailing" .)