About 50, one expert is trustworthy (8)
I believe the answer is:
reliable
'trustworthy' is the definition.
(similar in meaning)
'about 50 one expert' is the wordplay.
I cannot quite see how this works, but
'about' could be 're' (regarding) and 're' is found in the answer.
'50' could be 'l' (Roman numeral) and 'l' is found within the answer.
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
'expert' could be 'able' (able can mean expert or talented) and 'able' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a link.
Can you help me to learn more?
(Other definitions for reliable that I've seen before include "Dependable, honest" , "Unfailing" , "Can be depended upon, trustworthy" , "Faithful" , "Dependable, trustworthy" .)