Scottish cross-country runner (3)
I believe the answer is:
tay
'runner' is the definition.
(Scottish river)
'scottish cross-country' is the wordplay.
I cannot really see how this works, but
'cross' could be 't' (shape) and 't' is found within the answer.
The remaining letters 'ay' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tay that I've seen before include "Wicklow lake" , "Scotland's longest river" , "Dundee's river" , "Longest Scottish river" , "Longest river of Scotland" .)