Key phase, outwardly expensive (5)
I believe the answer is:
steep
'expensive' is the definition.
(I've seen this before)
'key phase outwardly' is the wordplay.
'key' becomes 'e' (musical key).
'phase' becomes 'step' (I've seen this in another clue).
'outwardly' is an insertion indicator.
'e' going into 'step' is 'STEEP'.
(Other definitions for steep that I've seen before include "Rapidly rising" , "not reasonable" , "over the top" , "Soak; expensive (price)" , "Sharply inclined" .)