Cross out tips from optimist (4)
I believe the answer is:
omit
'cross out' is the definition.
(thesaurus)
'tips from optimist' is the wordplay.
'from' indicates named letters should be taken away.
'optimist' with 'tips' removed is 'OMIT'.
(Other definitions for omit that I've seen before include "Leave out, do not include (4)" , "Fail to mention" , "Leave out or exclude" , "Don't include" , "Leave something out" .)