Holy person holds one position (3)
I believe the answer is:
sit
'position' is the definition.
(sitting is a kind of positioning)
'holy person holds one' is the wordplay.
'holy person' becomes 'st' (short for saint).
'holds' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'st' enclosing 'i' is 'SIT'.
(Other definitions for sit that I've seen before include "Perch" , "Serve (on jury)" , "Order dog to behave" , "as magistrates do" , "To take the test" .)