Headstrong, like horse at end of run (4)
I believe the answer is:
rash
'headstrong' is the definition.
(I've seen this in another clue)
'like horse at end of run' is the wordplay.
'like' becomes 'as'.
'horse' becomes 'h' (synonyms).
'at end of' means one lot of letters go next to another (some letters go at the end of others).
'run' becomes 'r' (cricket abbreviation).
'as'+'h'='ash'
'ash' put after 'r' is 'RASH'.
(Other definitions for rash that I've seen before include "Flood" , "Impulsive, having skin ailment" , "Overhasty - skin irritation" , "complaint" , "Headstrong" .)