Take off, and cross summit (4)
I believe the answer is:
apex
'summit' is the definition.
(I've seen this in another clue)
'take off and cross' is the wordplay.
'take off' becomes 'ape' (both can mean to imitate).
'and' means one lot of letters go next to another.
'cross' becomes 'x' (X is cross-shaped).
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "crown" , "climax" , "The tip or highest point (4)" , "Point or summit" , "high-point" .)