Earl employed to cover up one showing signs of old age? (4-6)

'showing signs of old age?' is the definition.
(grey hair is a sign of old age)

'earl employed to cover up one' is the wordplay.
'earl' becomes 'Grey' (as in Earl Grey, h9th-century prime minister).
'employed' becomes 'hired' ('hire' can be a synonym of 'employ').
'to cover up' means one lot of letters goes inside another.
'one' becomes 'a' (a thing is one thing).
'hired' placed around 'a' is 'haired'.
'grey'+'haired'='GREY-HAIRED'