Is, alas, preparing to attack (6)
I believe the answer is:
assail
'attack' is the definition.
(synonyms)
'is alas preparing' is the wordplay.
'preparing' indicates an anagram (the same letters prepared differently).
'is'+'alas'='isalas'
'isalas' with letters rearranged gives 'ASSAIL'.
'to' is the link.
(Other definitions for assail that I've seen before include "Maybe mug" , "Molest or harass" , "Aggressively attack" , "Slate" , "Beset" .)