Is Diana extremely suspicious? Does help (4)
I believe the answer is:
aids
'does help' is the definition.
('aid' can be a synonym of 'help')
'is diana extremely suspicious?' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'suspicious?' is an anagram indicator (I've seen this in other clues).
'diana' with its centre removed is 'da'.
'is'+'da'='isda'
'isda' is an anagram of 'AIDS'.
(Other definitions for aids that I've seen before include "Comes to the rescue" , "Helps with a dread disease" , "Said to be a modern plague" , "Assists" , "Said to be an international disease" .)