Remark’s caught you shouldn’t have to write down (6)
I believe the answer is:
notate
'write down' is the definition.
(I've seen this in another clue)
'remark's caught you shouldn't have' is the wordplay.
'remark' becomes 'note' (synonyms).
'caught' means one lot of letters goes inside another.
'you shouldn't have' becomes 'ta' (both can mean 'thank you').
'note' placed around 'ta' is 'NOTATE'.
'to' acts as a link.
(Other definitions for notate that I've seen before include "Write using signs or symbols as in music etc" , "Put down in symbols" , "At Eton (anag.)" , "Write in (musical) symbols" , "Write symbolically" .)