Tip of cross covered by primate (4)
I believe the answer is:
apex
'tip' is the definition.
(I've seen this before)
'cross covered by primate' is the wordplay.
'cross' becomes 'x' (X is cross-shaped).
'covered by' means one lot of letters go next to another (on top of in a down clue).
'primate' becomes 'ape' (large primate).
'x' after 'ape' is 'APEX'.
'of' is the link.
(Other definitions for apex that I've seen before include "Summit - pinnacle" , "Cheap fare" , "Culmination" , "Pointed top" , "climax" .)