On the tours of Trinity (5)
I believe the answer is:
three
'trinity' is the definition.
('three' can be a synonym of 'trinity')
'on the tours' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'tours' means one lot of letters goes inside another.
're' put inside 'the' is 'THREE'.
'of' acts as a link.
(Other definitions for three that I've seen before include "Number constituting a crowd?" , "Sixth root of seven hundred and twenty nine" , "Number of strikes before you're out" , "Trinity, triad" , "III" .)