It's well for one's superior (6)
I believe the answer is:
better
This might be a double definition.
'it's well' is the first definition.
(similar in meaning)
'one's superior' is the second definition.
'better' can be an answer for 'superior' (better is a kind of superior). I am not certain of the 'one's' bit.
'for' is the link.
Can you help me to learn more?
(Other definitions for better that I've seen before include "Top" , "Preferable" , "person making use of 43 [TOTE]" , "More desirable; gambler" , "Partly or fully recovered from illness" .)