Old clubs song about Newton's instrument (7)
I believe the answer is:
ocarina
'instrument' is the definition.
(ocarina is a kind of instrument)
'old clubs song about newton's' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'clubs' becomes 'c' (abbreviation used in card games such as bridge).
'song' becomes 'aria' (piece of music in an opera).
'about' means one lot of letters goes inside another.
'newton' becomes 'N' (symbol for the scientific unit of force).
'aria' placed around 'n' is 'arina'.
'o'+'c'+'arina'='OCARINA'
(Other definitions for ocarina that I've seen before include "we blow it" , "Bird-shaped flute" , "it's blown in pieces" , "Terracotta wind instr." , "source of music" .)