Hint: posh bloke invests one penny (3-3)
I believe the answer is:
tip-off
'hint' is the definition.
(thesaurus)
'posh bloke invests one penny' is the wordplay.
'posh bloke' becomes 'toff' (informal term for an upper class person).
'invests' means one lot of letters goes inside another (inserted letters are invested).
'one' becomes 'i' (Roman numeral).
'penny' becomes 'p' (short for pence).
'i'+'p'='ip'
'toff' placed around 'ip' is 'TIP-OFF'.
(Other definitions for tip-off that I've seen before include "Confidential warning" , "Give advance warning or inside information" , "Piece of secret information" , "Inside information given" , "Give advance warning or hint" .)