Terrible pain with old instrument (5)
I believe the answer is:
piano
'instrument' is the definition.
(musical instrument)
'terrible pain with old' is the wordplay.
'terrible' indicates an anagram.
'with' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'pain' anagrammed gives 'pian'.
'pian'+'o'='PIANO'
(Other definitions for piano that I've seen before include "p (mus.)" , "Instrument with ivories" , "In musical notation, this means softly" , "entertain with this softly" , "Quiet (music)" .)