Regret loss of earth washed off mountains (5)
I believe the answer is:
mourn
'regret loss' is the definition.
'earth washed off mountains' is the wordplay.
'earth' becomes 'E'.
'washed off' suggests deleting specific letters.
'mountains' becomes 'Mourne' (Mountains of Mourne in Northern Ireland).
'mourne' with 'e' taken away is 'MOURN'.
'of' acts as a link.
(Other definitions for mourn that I've seen before include "Grieve for a death" , "Show sorrow for loss" , "Keen" , "Regret (a loss)" , "'Miss'" .)