... Frank may be the son going astray (6)
I believe the answer is:
honest
'frank' is the definition.
(similar in meaning)
'the son going astray' is the wordplay.
'going astray' indicates anagramming the letters.
'the'+'son'='theson'
'theson' with letters rearranged gives 'HONEST'.
'may be' acts as a link.
(Other definitions for honest that I've seen before include "Honorable, true" , "Not lying" , "Truthful and trustworthy" , "Fair and just, not crooked" , "Truthful and dependable" .)