They will, of course, be themselves (4)
I believe the answer is:
boys
'themselves' is the definition.
Both the definition and answer are plural nouns.
Perhaps there's an association between them I don't understand?
'they will of course' is the wordplay.
I cannot really understand how this works, but
'of' could be 'o' and 'o' is found in the answer.
'course' could be 's' and 's' is found in the answer.
The remaining letters 'by' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'be' is the link.
Can you help me to learn more?
(Other definitions for boys that I've seen before include "Male children" , "Lads" , "They will be themselves, it's said" , "Young males" , "Young men" .)