Following the cross in ten up (4)
I believe the answer is:
next
'following' is the definition.
('next' can be a synonym of 'following')
'the cross in ten up' is the wordplay.
'the cross' becomes 'x' (**).
'in' means one lot of letters goes inside another.
'up' is a reversal indicator (in down clue: letters go upwards).
'x' placed within 'ten' is 'txen'.
'txen' backwards is 'NEXT'.
(Other definitions for next that I've seen before include "Nearest in space or position" , "Succeeding" , "Retailer" , "Coming nearest in time or sequence" , "Subsequent" .)