Tenth time he’s following it up (5)
I believe the answer is:
tithe
'tenth' is the definition.
(a tenth of one's income given to a church)
'time he's following it up' is the wordplay.
'time' becomes 't' (abbreviation).
'following' means one lot of letters go next to another.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'it' in reverse letter order is 'ti'.
't'+'he'='the'
'the' put after 'ti' is 'TITHE'.
(Other definitions for tithe that I've seen before include "One tenth - old church tax" , "Tax paid to church" , "It used to decimate income" , "One-tenth of income paid to church" , "10% tax" .)