re the ''O''---(1st letter of 3 letter word)
the clue was that it wanted a 3 letter word which would link with each of these 3 words---
Belt, Shaft, & River
for some reason I put OFF, but think thats probably wrong ! maybe its Cam, therefore giving
RACQUET.
So thanks all, the answer must be RACQUET
(after all it is the tennis season !