Is unable to tip (4)
I believe the answer is:
cant
I believe this is a double definition.
'is unable to' is the first definition.
(can not)
'tip' is the second definition.
(thesaurus)
(Other definitions for cant that I've seen before include "of words with specialised meaning" , "Pious claptrap" , "make a lot pay?" , "List" , "Stock phrases made nonsensical by endless repetition" .)