Mostly desire short type of tree (6)
I believe the answer is:
willow
'type of tree' is the definition.
'mostly desire short' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'desire' becomes 'will' (I've seen this in another clue).
'short' becomes 'low' (associated in meaning).
'will' with its final letter taken away is 'wil'.
'wil'+'low'='WILLOW'
(Other definitions for willow that I've seen before include "Tree providing wood for cricket bats" , "Fast-growing tree" , "Osier; cricket bat" , "Pussy- - -" , "Common riverside tree" .)