Exclamation, making mistake with croquet equipment (6)
I believe the answer is:
whoops
'exclamation making mistake' is the definition.
(exclamation after a making mistake)
'with croquet equipment' is the wordplay.
'with' becomes 'w' (abbreviation).
'croquet equipment' becomes 'hoops' (hoops are used in croquet).
'w'+'hoops'='WHOOPS'
(Other definitions for whoops that I've seen before include "Oh dear! That was a mistake!" , "Worrying cry" , "Exclamation after making a mistake" , "Something said after minor accident or mistake" , "Exclamation after a mistake; wild cries" .)