Manage to secure head of stiff brush (5)
I believe the answer is:
copse
'brush' is the definition.
('copse' can be a synonym of 'brush')
'manage to secure head of stiff' is the wordplay.
'manage' becomes 'cope' ('cope' can be a synonym of 'manage'**).
'to secure' is an insertion indicator.
'head of' suggests taking the first letters.
The first letter of 'stiff' is 's'.
'cope' placed around 's' is 'COPSE'.
(Other definitions for copse that I've seen before include "Dense thicket of trees" , "Cluster of trees" , "Coppice" , "Clump of small trees for regular cropping" , "A little wood" .)