Ageing tree starts to look yellowish (7)
I believe the answer is:
Here is my best explanation:
'ageing' is the definition.
'tree starts to look yellowish' is the subsidiary indication.
'tree' becomes 'elder' (elder is a kind of tree**).
'starts to' indicates taking the first letters.
The initial letters of 'look yellowish' is 'ly'.
(Other definitions for elderly that I've seen before include "over the hill", "Long in the tooth", "Quite old", "Of advanced years", "Mature", "Not young", "getting on in years", "Very senior?", "Past middle age, a senior citizen" and "Fairly old, past middle age".)