Old copper goes after two policemen start to suspect one hasn't been seen in ages (10)

'start to suspect one hasn't been seen in ages' is the definition.
I can't judge whether this definition defines the answer.

'old copper goes after two policemen' is the wordplay.
'two policemen' can mean two substitutions for 'policeman'.
'old copper' becomes 'ocus' (I can't explain this - if you can you should give a lot more credence to this answer).
'goes after' means one lot of letters go next to another.
'policeman' becomes 'di' (abbreviation for Detective Inspector).
'policeman' becomes 'plod' (UK informal term for a police officer).
'di'+'plod'='diplod'
'ocus' after 'diplod' is 'DIPLODOCUS'.

Can you help me to learn more?