Really wrong to acquire celery mix (9)
I believe the answer is:
sincerely
'really' is the definition.
(I've seen this before)
'wrong to acquire celery mix' is the wordplay.
I cannot really see how this works, but
'wrong' could be 'sin' (wrong can mean a sin or crime) and 'sin' is located in the answer.
an anagram of 'celery' is 'cerely' which is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sincerely that I've seen before include "Not showing any deceit" , "Really" , "In a genuine way" , "Genuinely, honestly" , "the last word before signing" .)