Tip that made the beast cross (4)
I believe the answer is:
apex
'tip that' is the definition.
'apex' can be an answer for 'tip' (thesaurus). I am not sure about the remainder of the definition.
'the beast cross' is the wordplay.
'the beast' becomes 'ape' (ape is a kind of beast).
'cross' becomes 'x'.
'ape'+'x'='APEX'
'made' is the link.
Can you help me to learn more?
(Other definitions for apex that I've seen before include "The highest point or pointed end" , "crown" , "Pointed top" , "climax" , "Summit - pinnacle" .)