Unwisely rely on instrument (4)
I believe the answer is:
lyre
'instrument' is the definition.
(lyre is a kind of instrument)
'unwisely rely' is the wordplay.
'unwisely' indicates anagramming the letters.
'rely' with letters rearranged gives 'LYRE'.
'on' is the link.
(Other definitions for lyre that I've seen before include "Ancient Greece harp" , "Ancient Greek harp" , "Instrument played by Orpheus" , "used to produce music" , "Old musical instrument with strings in U-shaped frame" .)