Trinity right within you (5)
I believe the answer is:
three
'trinity' is the definition.
('three' can be a synonym of 'trinity')
'right within you' is the wordplay.
'right' becomes 'r' (common abbreviation).
'within' indicates putting letters inside.
'you' becomes 'thee' (I've seen this in other clues).
'r' inserted into 'thee' is 'THREE'.
(Other definitions for three that I've seen before include "Number of blind mice" , "There (anag.)" , "Number of feet in a yard" , "Number of Dumas's musketeers" , "... number of his "Steps to Heaven"" .)