Prepare an ambush as I tie in-law in knots (3,2,4)
I believe the answer is:
lie in wait
'prepare an ambush' is the definition.
(I've seen this before)
'i tie in-law in knots' is the wordplay.
'in knots' is an anagram indicator (I've seen this in other clues).
'i'+'tie'+'in'+'law'='itieinlaw'
'itieinlaw' with letters rearranged gives 'LIE IN WAIT'.
'as' acts as a link.
(Other definitions for lie in wait that I've seen before include "remain hidden while preparing to attack" , "Hope to catch" , "Be prepared to ambush" , "Bide time" , "what will hunter do?" .)