Sappers apt to be trustworthy (8)
I believe the answer is:
reliable
'be trustworthy' is the definition.
('reliable' is associated in meaning with 'trustworthy')
'sappers apt' is the wordplay.
'sappers' becomes 're' (Royal Engineers).
'apt' becomes 'liable' ('liable' can be a synonym of 'apt'**).
're'+'liable'='RELIABLE'
'to' acts as a link.
(Other definitions for reliable that I've seen before include "Able to be depended on" , "Unfailing" , "Faithful" , "Trustworthy - genuine" , "Can be depended upon, trustworthy" .)