Wants to be seen turning the key in it (5)
I believe the answer is:
needs
'wants' is the definition.
(needing is a kind of wanting)
'seen turning the key in it' is the wordplay.
'turning' is a reversal indicator.
'the key' becomes 'd' (musical key).
'in it' indicates putting letters inside.
'seen' written backwards gives 'nees'.
'nees' enclosing 'd' is 'NEEDS'.
'to be' acts as a 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" .)