Run “Roundhead” in the race (4)
I believe the answer is:
trot
'race' is the definition.
(thesaurus)
'run roundhead in the' is the wordplay.
'run' becomes 'tot' (I can't explain this - if you can you should give a lot more credence to this answer).
'roundhead' becomes 'r' (head letter of 'round').
'in the' is an insertion indicator.
'tot' placed around 'r' is 'TROT'.
Can you help me to learn more?
(Other definitions for trot that I've seen before include "leftie" , "Left wing" , "Slowish run, of horse or person" , "Old communist" , "movement of H [TROT]" .)