Secure base of angel on top of tree (4)
I believe the answer is:
lash
'secure' is the definition.
(thesaurus)
'base of angel on top of tree' is the wordplay.
'base of' says to take the final letters.
'on top of' means one lot of letters go next to another.
'tree' becomes 'ash' (ash is a kind of tree).
The final letter of 'angel' is 'l'.
'l'+'ash'='LASH'
(Other definitions for lash that I've seen before include "Beat; fasten with cord" , "Strike with a whip" , "Fasten with rope; whip" , "Whip; securely fasten" , "Whip; attack verbally" .)