The lb50 is back on the shelf (4)
I believe the answer is:
sill
'shelf' is the definition.
(I've seen this before)
'the lb50 is back on' is the wordplay.
'the lb50' becomes 'll' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'back' is a reversal indicator.
'on' says to put letters next to each other.
'is' in reverse letter order is 'si'.
'll' put after 'si' is 'SILL'.
'the' is the link.
Can you help me to learn more?
(Other definitions for sill that I've seen before include "Shelf at the foot of a window" , "Horizontal timber forming the bottom of a framework" , "Window edge" , "Underwater rock ledge" , "Slab at foot of window" .)