Secure one working cogwheel (6)
I believe the answer is:
pinion
'cogwheel' is the definition.
(pinion is a kind of cogwheel)
'secure one working' is the wordplay.
'secure' becomes 'pin' (pinning is a kind of securing).
'one' becomes 'i' (Roman numeral).
'working' becomes 'on' (e.g. a machine is working or on).
'pin'+'i'+'on'='PINION'
(Other definitions for pinion that I've seen before include "shackle" , "Immobilize" , "Cogwheel; seize and secure (arms)" , "Wing; bind; cog" , "Cogwheel; part of wing" .)