Manage to fence small thicket (5)
I believe the answer is:
copse
'thicket' is the definition.
(synonyms)
'manage to fence small' is the wordplay.
'manage' becomes 'cope' (synonyms).
'to fence' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cope' placed around 's' is 'COPSE'.
(Other definitions for copse that I've seen before include "Coppice" , "A little wood" , "Small group of trees" , "Dense growth of shrubs or trees" , "Dense thicket of trees" .)