Lad outside Post Office is stirrer (5)
I believe the answer is:
spoon
'is stirrer' is the definition.
(I've seen this in another clue)
'lad outside post office' is the wordplay.
'lad' becomes 'son' (lad can informally mean a young boy).
'outside' indicates putting letters inside.
'post office' becomes 'po' (abbreviation).
'son' going around 'po' is 'SPOON'.
(Other definitions for spoon that I've seen before include "'He that sups with the devil should have a long . . . . .'" , "Ancient club" , "One for taking soup" , "Piece of cutlery" , "fellow eloper during high jump event" .)