There's a lot to be said for being ethical (5)
I believe the answer is:
moral
'being ethical' is the definition.
(synonyms)
'there's a lot to be said' is the wordplay.
'there's a lot' becomes 'm' ().
'to be said' becomes 'oral' (I've seen this before).
'm'+'oral'='MORAL'
'for' acts as a link.
(Other definitions for moral that I've seen before include "Lesson offered by story" , "Practical lesson to be drawn from an experience" , "Lesson that can be drawn from a story" , "Underlying significance of a story or event" , "Honourable" .)