Where being over-prudent may cost you a lot (7,4)

'where' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'over-prudent may cost you a lot' is the wordplay.
I cannot quite see how this works, but
'over' could be 'on' (physically on top of) and 'on' is found within the answer.
'you' could be 'u' (homophones) and 'u' is found within the answer.
'a' is present in the answer.
'lot' could be 'm' and 'm' is found within the answer.
This may be the basis of the clue (or it may be nonsense).

'being' acts as a link.

Can you help me to learn more?