Development in love, out at sea (9)
I believe the answer is:
evolution
'development' is the definition.
(I know that development can be written as evolution)
'in love out at sea' is the wordplay.
'at sea' indicates an anagram.
'in'+'love'+'out'='inloveout'
'inloveout' anagrammed gives 'EVOLUTION'.
(Other definitions for evolution that I've seen before include "Development - progress" , "Gradual development" , "Darwinian process" , "it's a gradual process" , "Darwinian theory" .)