Sailor at end of cavern finding mountain lake (4)
I believe the answer is:
tarn
'mountain lake' is the definition.
(I've seen this before)
'sailor at end of cavern' is the wordplay.
'sailor' becomes 'tar' (slang term for a sailor).
'at end of' suggests the final letters.
The final letter of 'cavern' is 'n'.
'tar'+'n'='TARN'
'finding' acts as a link.
(Other definitions for tarn that I've seen before include "High water" , "Scottish mountain lake" , "Small mountain lake" , "Mountain pool: French department" .)