Move slowly inside church (4)
I believe the answer is:
inch
'move slowly' is the definition.
(to inch along is to move slowly)
'inside church' is the wordplay.
'inside' becomes 'in'.
'church' becomes 'ch' (abbreviation on maps etc).
'in'+'ch'='INCH'
(Other definitions for inch that I've seen before include "Advance slowly" , "Scottish island: unit of length" , "... or not so long < , Celtic island" , "One of a 2 down in a foot" , "Linear measure" .)