Catch southern horse (4)
I believe the answer is:
snag
'catch' is the definition.
(snagging is a kind of catching)
'southern horse' is the wordplay.
'southern' becomes 's' (abbreviation).
'horse' becomes 'nag' (nag is a kind of horse).
's'+'nag'='SNAG'
(Other definitions for snag that I've seen before include "banger down under" , "Jag" , "Hidden obstacle or catch" , "hurdle?" , "Unexpected difficulty" .)