Mounting snow regularly holds up work (4)
I believe the answer is:
opus
'work' is the definition.
(I've seen this before)
'mounting snow regularly holds up' is the wordplay.
'mounting' says the letters should be written backwards (going up in a down clue).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'holds' is an insertion indicator.
The alternating letters of 'snow' are 'so'.
'so' placed around 'up' is 'supo'.
'supo' reversed gives 'OPUS'.
(Other definitions for opus that I've seen before include "Artistic work" , "Work (of a composer?)" , "Work (mus.)" , "Piece of music (with number)" , "Major work" .)