Catch horse after start of steeplechase (4)
I believe the answer is:
snag
'catch' is the definition.
(snagging is a kind of catching)
'horse after start of steeplechase' is the wordplay.
'horse' becomes 'nag' (nag is a kind of horse).
'after' means one lot of letters go next to another.
'start of' indicates taking the first letters.
The first letter of 'steeplechase' is 's'.
'nag' put after 's' is 'SNAG'.
(Other definitions for snag that I've seen before include "banger down under" , "Sang about obstacle" , "Complication, drawback" , "hurdle?" , "Unexpected difficulty" .)