Costs maintaining essentially expensive royal offspring (7)
I believe the answer is:
princes
'costs maintaining essentially expensive royal offspring' is the definition.
I can't judge whether this defines the answer.
'costs maintaining essentially expensive royal offspring' is the wordplay.
'costs' becomes 'prices' ('price' can be a synonym of 'cost').
'maintaining' indicates putting letters inside.
'essentially expensive royal offspring' becomes 'n' (I can't explain this - if you can you should give a lot more credence to this answer).
'prices' going around 'n' is 'PRINCES'.
Can you help me to learn more?
(Other definitions for princes that I've seen before include "Male royals" , "king's followers?" .)