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' is an anagram indicator.
'rely' with letters rearranged gives 'LYRE'.
'on' acts as a 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" .)