Instrument to mend stair (5)
I believe the answer is:
sitar
'instrument' is the definition.
(musical instrument)
'mend stair' is the wordplay.
'mend' indicates an anagram (I've seen 'mended' mean this).
'stair' anagrammed gives 'SITAR'.
'to' acts as a link.
(Other definitions for sitar that I've seen before include "music producer" , "music-maker" , "Indian string instrument" , "instrument out east" , "Guitar-like instrument" .)