A duke and daughter happy to be consistent (3,2)
I believe the answer is:
add up
'be consistent' is the definition.
(add up means to be consistent or logical)
'a duke and daughter happy' is the wordplay.
'duke' becomes 'd' (abbreviation).
'and' is a charade indicator (letters next to each other).
'daughter' becomes 'd' (genealogical abbreviation).
'happy' becomes 'up' (I've seen this in other clues).
'a'+'d'+'d'+'up'='ADD UP'
'to' acts as a link.
(Other definitions for add up that I've seen before include "Aggregate, get total" , "Put together (numbers)" , "Seem consistent or reasonable - arithmetically?" , "Seem to be sensible" , "Make sense; count" .)