Mark's boy holding onto old record company pass (9)
I believe the answer is:
semicolon
'mark's' is the definition.
(semicolon is a kind of punctuation mark)
'boy holding onto old record company pass' is the wordplay.
'boy' becomes 'son' (synonyms).
'holding' indicates putting letters inside.
'onto' means one lot of letters go next to another.
'old record company' becomes 'EMI' (record company broken up in o0so).
'pass' becomes 'col' (col is a term for a mountain pass).
'emi'+'col' is 'emicol'.
'son' enclosing 'emicol' is 'SEMICOLON'.
(Other definitions for semicolon that I've seen before include "Joiner" , "used by writers for a break" , "Punctuation mark denoting a pause in a sentence" , "Item of punctuation" , "half a stop" .)