Let it stand for hat son mislaid (4)
I believe the answer is:
stet
'let it stand' is the definition.
(I've seen this in another clue)
'hat son mislaid' is the wordplay.
'hat' becomes 'stetson' (Stetson is an example).
'mislaid' suggests deleting specific letters.
'stetson' with 'son' taken out is 'STET'.
'for' acts as a link.
(Other definitions for stet that I've seen before include "(On proofs) instruction to cancel a correction" , "Eg, molar" , "Notation meaning as is, thus" , "Direction in printing to cancel a correction or deletion" , "Notation meaning leave as is, don't change spelling" .)