Take off before 'Number Ten' summit (4)
I believe the answer is:
apex
'summit' is the definition.
(thesaurus)
'take off before number ten' is the wordplay.
'take off' becomes 'ape' (I've seen this before).
'before' means one lot of letters go next to another.
'number ten' becomes 'x' (Roman numeral).
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "Pointed top" , "Culmination" , "high-point" , "Uppermost tip" , "Top point" .)