Security device for motorcycle triallist (9)
I believe the answer is:
scrambler
I believe this is a double definition.
'security device' is the first definition.
(I've seen this before)
'motorcycle triallist' is the second definition.
(scrambler is a type of motorcycle)
'for' acts as a link.
(Other definitions for scrambler that I've seen before include "muddler?" , "one on motorcycle" , "Telephone security device" , "One hurrying to take off" , "Biker" .)