Lake bird ultimately happier on home ground (7)
I believe the answer is:
moorhen
'lake bird' is the definition.
'ultimately happier on home ground' is the wordplay.
'ultimately' suggests the final letters.
'ground' indicates an anagram (grind the letters ).
The final letter of 'happier' is 'r'.
'r'+'on'+'home'='ronhome'
'ronhome' anagrammed gives 'MOORHEN'.
(Other definitions for moorhen that I've seen before include "Female red grouse or waterfowl" , "Water bird" , "Medium-sized, blackish aquatic bird with long toes" , "pond bird" , "Black freshwater bird" .)