Take off before ten for summit (4)
I believe the answer is:
apex
'summit' is the definition.
(thesaurus)
'take off before ten' is the wordplay.
'take off' becomes 'ape' (I've seen this before).
'before' means one lot of letters go next to another.
'ten' becomes 'x' (roman numeral).
'ape'+'x'='APEX'
'for' acts as a link.
(Other definitions for apex that I've seen before include "The tip or highest point (4)" , "Topmost point" , "The highest point of something" , "The highest point or pointed end" , "Point or summit" .)