Start putting on clothes for sport (11)
I believe the answer is:
showjumping
'sport' is the definition.
(showjumping is a kind of sport)
'start putting on clothes' is the wordplay.
'start' becomes 'jump' (jumping is a kind of starting).
'putting on' becomes 'showing'.
'clothes' is an insertion indicator.
'jump' going inside 'showing' is 'SHOWJUMPING'.
'for' is the link.
(Other definitions for showjumping that I've seen before include "Equestrian sport" , "Equestrian event" , "sports event" , "Equestrian contest with fences etc" , "Riding competition" .)