Fissure allows the French inside underground chamber (6)
I believe the answer is:
cleave
'fissure' is the definition.
(I've seen this before)
'the french inside underground chamber' is the wordplay.
'the french' becomes 'le'.
'inside' is an insertion indicator.
'underground chamber' becomes 'cave' (I've seen this before).
'le' placed inside 'cave' is 'CLEAVE'.
'allows' is the link.
(Other definitions for cleave that I've seen before include "Stick fast; split" , "To separate and to adhere" , "Part" , "Chop or split" , "Split apart - stick together" .)