Nevertheless, it's put up by £50 (5)
I believe the answer is:
still
'nevertheless' is the definition.
(I know that nevertheless can be written as still)
'it's put up by ps50' is the wordplay.
'put up' shows that the letters should be reversed in order.
'by' is a charade indicator (letters next to each other).
'ps50' becomes 'ill' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ts' reversed gives 'st'.
'st'+'ill'='STILL'
Can you help me to learn more?
(Other definitions for still that I've seen before include "Unmoving yet" , "Not going anywhere" , "Motionless - nevertheless" , "not bubbly" , "Where to make spirits" .)