Living things turning brown, certainly missing sun (6)
I believe the answer is:
nature
'living' is the definition.
The answer and definition can be both states of affairs as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'turning brown certainly missing sun' is the wordplay.
'turning brown' becomes 'nat' (I can't justify this - if you can you should give a lot more credence to this answer).
'certainly' becomes 'sure' (synonyms).
'missing' indicates named letters should be taken away.
'sun' becomes 'S' (abbreviation).
'sure' with 's' removed is 'ure'.
'nat'+'ure'='NATURE'
'things' is the link.
Can you help me to learn more?
(Other definitions for nature that I've seen before include "Basic character" , "Tea-urn for outdoors" , "(Person's) character" , "- Study; - reserve" , "Character, make-up" .)