Looking sickly, paid about lb50 (6)
I believe the answer is:
pallid
'looking sickly' is the definition.
I can't tell whether this defines the answer.
'paid about lb50' is the wordplay.
'about' means one lot of letters goes inside another.
'lb50' becomes 'll' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'paid' enclosing 'll' is 'PALLID'.
Can you help me to learn more?
(Other definitions for pallid that I've seen before include "Pale complexion because of poor health" , "Feeble, insipid" , "Insipid; lacking colour" , "Without healthy colour" , "White in face as from illness" .)