... Frank may be the son going astray (6)
I believe the answer is:
'frank' is the definition.
(similar in meaning)
'the son going astray' is the wordplay.
'going astray' is an anagram indicator.
'theson' is an anagram of 'HONEST'.
'may be' acts as a link.
(Other definitions for honest that I've seen before include "See 1", "Virtuous", "Law-abiding", "Genuine and trustworthy", "really!".)