Catching rook, cat — the only one in Disney — makes start (7)
I believe the answer is:
trigger
'start' is the definition.
(I've seen this before)
'catching rook cat the only one in disney' is the wordplay.
I cannot really understand how this works, but
'rook' could be 'r' (abbreviation in chess) and 'r' is located in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
'one' could be 'er' (I've seen this in other clues) and 'er' is present in the answer.
This explanation may well be incorrect...
'makes' is the link.
Can you help me to learn more?
(Other definitions for trigger that I've seen before include "Roy Rogers' horse" , "Gun firing device" , "Gun-firing device" , "Start" , "Equine film star" .)