Answer: fissure

AnswerCrossword Clue
FISSURE

Narrow opening

FISSURE

Rift

FISSURE

Organ groove

FISSURE
Narrow gap
FISSURE
Crack
FISSURECleft in a rock.
FISSURESplit.
FISSURENarrow opening.
FISSURECleft
FISSURELava comes out of it
FISSURELong, narrow crack
FISSURECleavage
FISSURECerebral groove