More sneaky one hidden by small duck, almost there (10)

'more sneaky' is the definition.
('stealthy' can be a synonym of 'sneaky')

'one hidden by small duck almost there' is the wordplay.
'one' becomes 'i' (Roman numeral).
'hidden by' means one lot of letters goes inside another (inserted letters are hidden inside).
'small' becomes 's' (abbreviation - e.g. clothes size).
'duck' becomes 'teal' (teal is a type of duck).
'almost' means to remove the last letter (almost the whole word is kept).
'there' with its last letter taken off is 'ther'.
's'+'teal'+'ther'='stealther'
'i' put within 'stealther' is 'STEALTHIER'.