Stable and happy with little sister locked up (10)
I believe the answer is:
consistent
'stable' is the definition.
(I've seen this before)
'and happy with little sister locked up' is the wordplay.
'and' becomes 'n' (common abbreviation for 'and').
'happy with' indicates anagramming the letters (happy can mean slightly drunk).
'little' means to remove the last letter (the word is not quite complete).
'locked' means one lot of letters goes inside another (I've seen this in other clues).
'up' becomes 'cont' (I can't justify this - if you can you should give a lot more credence to this answer).
'sister' with its last letter removed is 'siste'.
'n' anagrammed with 'siste' can make 'sisten'.
'sisten' going inside 'cont' is 'CONSISTENT'.
Can you help me to learn more?
(Other definitions for consistent that I've seen before include "true to principles" , "in agreement" , "Steady" , "the same as always" , "Maintaining a standard over time" .)