Twister causes trouble to sailors at first (7)
I believe the answer is:
tornado
'twister' is the definition.
('tornado' can be a synonym of 'twister')
'trouble to sailors at first' is the wordplay.
'trouble' becomes 'ado' (I've seen this before).
'sailors' becomes 'rn' (abbreviation for Royal Navy).
'at first' is a charade indicator (letters next to each other).
'to'+'rn'='torn'
'ado' after 'torn' is 'TORNADO'.
'causes' acts as a link.
(Other definitions for tornado that I've seen before include "Violent twister" , "Typhoon" , "Funnelling wind" , "squall" , "Strong wind" .)