National leader veers off racecourse (4)
I believe the answer is:
scot
'national' is the definition.
(I've seen this before)
'leader veers off racecourse' is the wordplay.
'leader veers off' means to remove the first letter.
'racecourse' becomes 'ascot' (British racecourse).
'ascot' with its first letter taken off is 'SCOT'.
(Other definitions for scot that I've seen before include "A Caledonian?" , "Eg, a Glaswegian" , "Jock?" , "Stevenson was one" , "Aberdonian possibly" .)