Long-distance runner in Spain with nothing on (4)
I believe the answer is:
nile
'long-distance runner' is the definition.
'nile' can be an answer for 'runner' (I've seen this before). I'm not sure about the 'long-distance' bit.
'spain with nothing on' is the wordplay.
'spain' becomes 'e' (abbreviation for Espana).
'with' means one lot of letters go next to another.
'nothing on' becomes 'nil' (I can't explain this - if you can you should give a lot more credence to this answer).
'e' after 'nil' is 'NILE'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for nile that I've seen before include "Egypt's river" , "Current" , "Take a line to the river" , "River associated with Cleopatra" , "River that's Blue and White" .)