Silly poseurs have regularly demonstrated that they can be taken advantage of (9)
I believe the answer is:
pushovers
'that they can be taken advantage of' is the definition.
The definition suggests an adverb but the answer is not.
'silly poseurs have regularly' is the wordplay.
'silly' is an anagram indicator.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'have' are 'hv'.
'poseurs'+'hv'='poseurshv'
'poseurshv' anagrammed gives 'PUSHOVERS'.
'demonstrated' is the link.
Can you help me to learn more?
(Other definitions for pushovers that I've seen before include "piece of cake? More than one" , "Tries" .)