The last letter has it: her first stringed musical instruments (7)
I believe the answer is:
zithers
'musical instruments' is the definition.
(zither is a kind of musical instrument)
'the last letter has it her first stringed' is the wordplay.
'the last letter' becomes 'z' (in the alphabet).
'has' means one lot of letters go next to another.
'first stringed' becomes 's' (1st letter of 'stringed').
'z'+'it'+'her'+'s'='ZITHERS'
(Other definitions for zithers that I've seen before include "Stringed instruments played while laid flat" , "Stringed instruments plucked when lying flat" , "Note producers" , "Musical instruments" .)