Entrance blocked at front by a stone (5)
I believe the answer is:
agate
'stone' is the definition.
(I've seen this before)
'entrance blocked at front by a' is the wordplay.
'entrance' becomes 'gate' (I've seen this before).
'blocked at front by' says to put letters next to each other (some letters go in front of others).
'gate' after 'a' is 'AGATE'.
(Other definitions for agate that I've seen before include "Quartz variety" , "On the way" , "Quartz with bands of colour, used in jewellery" , "Semi-precious quartz" , "Impure form of quartz with banded chalcedony, used as a gemstone" .)