Ageing tree starts to look yellowish (7)
I believe the answer is:
elderly
'ageing' is the definition.
(thesaurus)
'tree starts to look yellowish' is the wordplay.
'tree' becomes 'elder' (elder is a kind of tree**).
'starts to' indicates taking the first letters.
The initial letters of 'look yellowish' is 'ly'.
'elder'+'ly'='ELDERLY'
(Other definitions for elderly that I've seen before include "Old or ageing" , "Past middle age, a senior citizen" , "Quite old" , "Fairly old, past middle age" , "over the hill" .)