The £50 is back on the shelf (4)
I believe the answer is:
sill
'the shelf' is the definition.
(I've seen this before)
'the ps50 is back on' is the wordplay.
'the ps50' becomes 'll' (I can't justify this - if you can you should believe this answer much more).
'back' shows that the letters should be reversed in order.
'on' means one lot of letters go next to another.
'is' back-to-front is 'si'.
'll' after 'si' is 'SILL'.
Can you help me to learn more?
(Other definitions for sill that I've seen before include "Underwater rock ledge" , "Window edge" , "Ledge beneath a window" , "Horizontal timber forming the bottom of a framework" , "Horizontal intrusion of igneous rock between sedimentaries, as in much of Hadrian's Wall" .)