Because engineers are worthy of credit (7)
I believe the answer is:
'worthy of credit' is the definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?
'because engineers' is the wordplay.
'because' becomes 'since' (I've seen this before).
'engineers' becomes 're' (abbreviation for Royal Engineers).
'are' is the link.
Can you help me to learn more?
(Other definitions for sincere that I've seen before include "like informal correspondents?", "Unfeigned, genuine", "straightforward", "Perfect", "Pure".)