Earl wearing trousers, tailored initially to be most loose (8)

'be most loose' is the definition.
('slack' can be a synonym of 'loose')

'earl wearing trousers tailored initially' is the wordplay.
'earl' becomes 'e'.
'wearing' means one lot of letters goes inside another.
'trousers' becomes 'slacks' (slacks is a kind of trousers**).
'initially' says to take the initial letters.
The initial letter of 'tailored' is 't'.
'e' inserted within 'slacks' is 'slackes'.
'slackes'+'t'='SLACKEST'

'to' is the link.