About 51, expert found to be trustworthy (8)
I believe the answer is:
reliable
'to be trustworthy' is the definition.
('reliable' is associated in meaning with 'trustworthy')
'about 51 expert' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'51' becomes 'LI' (Roman numerals).
'expert' becomes 'able' (able can mean expert or talented).
're'+'li'+'able'='RELIABLE'
'found' acts as a link.
(Other definitions for reliable that I've seen before include "Faithful" , "Able to be depended on" , "Dependable, can be trusted" , "Can be depended upon, trustworthy" , "Dependable, trustworthy" .)