At front of entrance there's a stone (5)
I believe the answer is:
agate
'there's a stone' is the definition.
(I've seen this before)
'at front of entrance' is the wordplay.
'front of' indicates taking the first letters.
'entrance' becomes 'gate' (I've seen this before**).
The first letter of 'at' is 'a'.
'a'+'gate'='AGATE'
(Other definitions for agate that I've seen before include "Hard stone with concentric bands of colour" , "Quartz variety" , "Impure form of quartz with banded chalcedony, used as a gemstone" , "Quartz with bands of colour, used in jewellery" , "Stone with coloured bands" .)