Wants to be seen turning the key in it (5)
I believe the answer is:
needs
'wants' is the definition.
('need' can be a synonym of 'want')
'seen turning the key in it' is the wordplay.
'turning' says the letters should be written backwards.
'the key' becomes 'd' (musical key).
'in it' indicates putting letters inside.
'seen' in reverse letter order is 'nees'.
'nees' enclosing 'd' is 'NEEDS'.
'to be' is the link.
(Other definitions for needs that I've seen before include "'..... must when the devil drives'" , "Requires, is in want of" , "Demands" , "Requirements" , "Things that are wanted" .)