People of refinement ultimately constrained by principles (7)
I believe the answer is:
mortals
'people' is the definition.
('mortal' can be a synonym of 'person')
'refinement ultimately constrained by principles' is the wordplay.
'refinement ultimately' becomes 't' (I am not sure about this - if you are sure you should believe this answer much more).
'constrained by' means one lot of letters goes inside another.
'principles' becomes 'morals' (I've seen this in another clue).
't' put within 'morals' is 'MORTALS'.
'of' is the link.
Can you help me to learn more?
(Other definitions for mortals that I've seen before include "People - lost arm (anag)" , "Lord, what fools these -- be! (AMND)" .)