Require key to get into flipping study! (4)
I believe the answer is:
need
'require' is the definition.
('need' can be a synonym of 'require')
'key to get into flipping study' is the wordplay.
'key' becomes 'e' (I've seen this in other clues**).
'to get into' indicates putting letters inside.
'flipping' says the letters should be written backwards.
'study' becomes 'den' (I've seen this before).
'den' written backwards gives 'ned'.
'e' going within 'ned' is 'NEED'.
(Other definitions for need that I've seen before include "must have results!" , "Require; want" , "Distressful situation" , "state of distress" , "Necessitate" .)