Instrument to fix stair (5)
I believe the answer is:
sitar
'instrument' is the definition.
(musical instrument)
'fix stair' is the wordplay.
'fix' indicates anagramming the letters.
'stair' with letters rearranged gives 'SITAR'.
'to' is the link.
(Other definitions for sitar that I've seen before include "Large Indian lute" , "Indian string instrument" , "Lasting from morning to night" , "Astir with Indian long-necked lute" , "Plucked instrument" .)