Holed in one side, catamaran partly capsized (4)
I believe the answer is:
aced
'holed in one' is the definition.
(I've seen this in another clue)
'side catamaran partly capsized' is the wordplay.
'partly' says the answer is hidden in the clue (part of the clue is used).
'capsized' shows that the letters should be reversed in order (in a down clue, the word turns upside down).
'deca' is hidden in the letters of 'side catamaran'.
'deca' back-to-front is 'ACED'.
(Other definitions for aced that I've seen before include "Comprehensively beaten" , "got a hole in one" , "passed with highest marks in US" , "Sailed through" , "Reached a golf hole in one shot" .)