Bet placed on unfashionable police operation (5-3)
I believe the answer is:
stake-out
'police operation' is the definition.
'bet placed on unfashionable' is the wordplay.
'bet' becomes 'stake' (staking is a kind of betting).
'placed on' means one lot of letters go next to another.
'unfashionable' becomes 'out' ('out' can be a synonym of 'unfashionable').
'stake'+'out'='STAKE-OUT'
(Another definition for stake-out that I've seen is " Police surveillance operation".)
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