Manage to enclose small group of trees (5)
I believe the answer is:
copse
'group of trees' is the definition.
(small woodland area)
'manage to enclose small' is the wordplay.
'manage' becomes 'cope' (manage can mean to cope or do well).
'to enclose' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cope' enclosing 's' is 'COPSE'.
(Other definitions for copse that I've seen before include "Dense thicket of trees" , "A little wood" , "Piece of managed woodland" , "Cluster of trees" , "Stand of trees" .)