There may well be money in backing it for 50lb (4)
I believe the answer is:
till
'there may well be money' is the definition.
I can't judge whether this defines the answer.
'backing it for 50lb' is the wordplay.
'backing' says the letters should be written backwards.
'for' is a charade indicator (letters next to each other).
'50lb' becomes 'll' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'it' in reverse letter order is 'ti'.
'ti'+'ll'='TILL'
'in' is the link.
Can you help me to learn more?
(Other definitions for till that I've seen before include "Up to (a point in time)" , "Dig soil" , "Cash found here" , "Container for money" , "cash container" .)