Thought son cut speed round the bend (9)
I believe the answer is:
suspected
'thought' is the definition.
(suspecting is a kind of thinking)
'son cut speed round the bend' is the wordplay.
'son' becomes 's'.
'round the bend' indicates an anagram.
'cut'+'speed'='cutspeed'
'cutspeed' with letters rearranged gives 'uspected'.
's'+'uspected'='SUSPECTED'
(Other definitions for suspected that I've seen before include "Imagined to be true" , "Fancied" , "mistrusted" , "Believed to be guilty but without proof" , "Believed (someone) guilty" .)