Fastener, one on tie (6)
I believe the answer is:
pinion
'tie' is the definition.
(thesaurus)
'fastener one on' is the wordplay.
'fastener' becomes 'pin' (pin is a kind of fastener).
'one' becomes 'i' (Roman numeral).
'pin'+'i'+'on'='PINION'
(Other definitions for pinion that I've seen before include "Wing of a bird" , "Restrain by holding, or bird's wing" , "shackle" , "Cogwheel; seize and secure (arms)" , "Immobilize" .)