Reject old Amercian college (4)
I believe the answer is:
omit
'reject' is the definition.
(thesaurus)
'old amercian college' is the wordplay.
I cannot really see how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is present in the answer.
'college' could be 'mit' (Massachusetts Institute of Technology) and 'mit' 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 omit that I've seen before include "Leave out, do not include (4)" , "Leave something out" , "Leave out or undone" , "Leave out, perhaps in error" , "Fail (to do something)" .)