Unwisely rely on an instrument (4)
I believe the answer is:
lyre
'an 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 "Sort of harp" , "Ancient harp-like instrument" , "Old musical instrument with strings in U-shaped frame" , "Ancient musical instrument - sounds mendacious" , "Music-maker" .)