Scottish leader with horse problem (4)
I believe the answer is:
snag
'problem' is the definition.
(snag can mean a problem or difficulty)
'scottish leader with horse' is the wordplay.
'leader' indicates taking the first letters.
'with' says to put letters next to each other.
'horse' becomes 'nag' (term for an old horse).
The first letter of 'scottish' is 's'.
's'+'nag'='SNAG'
(Other definitions for snag that I've seen before include "Complication, drawback" , "A hidden obstruction" , "Minor hitch" , "Minor difficulty" , "Minor problem" .)