Agitated, one possibly has to know! (6)
I believe the answer is:
shaken
'agitated' is the definition.
(similar in meaning)
'one possibly has to know' is the wordplay.
'one possibly' is an anagram indicator.
'to know' becomes 'ken' (I've seen this in another clue).
'has' with letters rearranged gives 'sha'.
'sha'+'ken'='SHAKEN'
(Other definitions for shaken that I've seen before include "Shocked or agitated" , "Upset - rattled" , "Shocked; brandished" , "Shocked; astonished" , "Agitated or joggled" .)