Trinity right inside you? (5)
I believe the answer is:
three
'trinity' is the definition.
(synonyms)
'right inside you?' is the wordplay.
'right' becomes 'r' (abbreviation).
'inside' means one lot of letters goes inside another.
'you?' becomes 'thee' (I've seen this before).
'r' inserted into 'thee' is 'THREE'.
(Other definitions for three that I've seen before include "Bears or blind mice" , "Cardinal number" , "Figure" , "Number of Dumas's musketeers" , "Number of Furies or Norns" .)