Dicky writes about onset of tragic tornado (7)
I believe the answer is:
twister
'tornado' is the definition.
('twister' can be a synonym of 'tornado')
'dicky writes about onset of tragic' is the wordplay.
'dicky' indicates an anagram (dicky can mean ill or non-functioning).
'about' indicates putting letters inside.
'onset of' suggests taking the first letters.
The initial letter of 'tragic' is 't'.
'writes' with letters rearranged gives 'twiser'.
'twiser' going around 't' is 'TWISTER'.
(Other definitions for twister that I've seen before include "Tornado (American)" , "Rogue - tornado" , "Violent local windstorm (US)" , "'sixties dancer?" , "A swindler or a cyclone" .)