Part of foot in church (4)
I believe the answer is:
inch
'part of foot' is the definition.
(12 inches in a foot)
'in church' is the wordplay.
'church' becomes 'ch' (abbreviation on maps etc).
'in'+'ch'='INCH'
(Other definitions for inch that I've seen before include "Twenty five point four millimetres" , "Scottish island: unit of length" , "The island is very small in length" , "Small Scottish island" , "Move along gradually" .)