Foot of deep snow mostly dispersed (7)
I believe the answer is:
spondee
'foot' is the definition.
(type of metrical foot in poetry)
'deep snow mostly dispersed' is the wordplay.
'mostly' means to remove the last letter.
'dispersed' is an anagram indicator.
'deep'+'snow'='deepsnow'
'deepsnow' with its final letter taken off is 'deepsno'.
'deepsno' anagrammed gives 'SPONDEE'.
'of' acts as a link.
(Other definitions for spondee that I've seen before include "bit of verse" , "Metric foot of two long syllables" , "No speed required for poetic metrical foot" .)