Start to cultivate mountain ash without a top (5)
I believe the answer is:
crown
'a top' is the definition.
(crown is a kind of top)
'start to cultivate mountain ash' is the wordplay.
'start' means to remove the last letter (I've seen 'starts' mean this).
'to cultivate' becomes 'crop' ('crop' can be a synonym of 'cultivate').
'mountain ash' becomes 'wn' (I am not sure about this - if you are sure you should believe this answer much more).
'crop' with its final letter taken away is 'cro'.
'cro'+'wn'='CROWN'
'without' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for crown that I've seen before include "sporting award" , "piece of tooth" , "Diadem" , "peak" , "Old coin; top of tree" .)