Sneak off but stop around back of castle in the style of William the First (5,4)

'sneak off' is the definition.
(I know that sneak off can be written as steal away)

'stop around back of castle in the style of william the first' is the wordplay.
'stop' becomes 'stay' (stay is a kind of stop).
'around' is an insertion indicator.
'back of' indicates one should take the final letters.
'in the style of' becomes 'a la' (e.g. chicken a la King).
'the first' says to take the initial letters.
The last letter of 'castle' is 'e'.
The initial letter of 'william' is 'w'.
'e'+'ala'+'w'='ealaw'
'stay' enclosing 'ealaw' is 'STEAL AWAY'.

'but' is the link.