Because engineers are worthy of credit (7)
I believe the answer is:
sincere
'worthy of credit' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I don't see?
'because engineers' is the wordplay.
'because' becomes 'since' ('since' can mean 'because').
'engineers' becomes 're' (abbreviation for Royal Engineers).
'since'+'re'='SINCERE'
'are' acts as a link.
Can you help me to learn more?
(Other definitions for sincere that I've seen before include "Free from deceit" , "Earnest" , "devout" , "Candid" , "Without deceit, genuine" .)