Thanks for the fifty quid - I can hardly believe it! (4)
I believe the answer is:
tall
'hardly believe it' is the definition.
I can't tell whether this defines the answer.
'thanks for the fifty quid i' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'for' means one lot of letters go next to another.
'the fifty quid i' becomes 'll' (I can't justify this - if you can you should give a lot more credence to this answer).
'ta'+'ll'='TALL'
'can' is the link.
Can you help me to learn more?
(Other definitions for tall that I've seen before include "High in stature" , "Bigger than average" , "Lofty; unlikely" , "Like giants" , "Unbelievable" .)