Deep fissure exploited by cavers and some seamen (8)
I believe the answer is:
crevasse
'deep fissure' is the definition.
(I've seen this before)
'exploited by cavers and some seamen' is the wordplay.
I cannot really see how this works, but
an anagram of 'cavers' is 'crevas' which is located in the answer.
The remaining letters 'se' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for crevasse that I've seen before include "Deep glacial fissure" , "13 in a glacier" , "opening" , "cold break" , "gap" .)