Take off, cross, for the summit (4)
I believe the answer is:
apex
'summit' is the definition.
(I've seen this before)
'take off cross' is the wordplay.
'take off' becomes 'ape' (I've seen this in other clues).
'cross' becomes 'x' (X is cross-shaped).
'ape'+'x'='APEX'
'for the' is the link.
(Other definitions for apex that I've seen before include "Acme" , "Culmination" , "high-point" , "Cheap fare" , "Top, peak" .)