Tree secure? Not entirely (3)
I believe the answer is:
fir
'tree' is the definition.
(fir is a kind of tree)
'secure? not entirely' is the wordplay.
'secure?' becomes 'firm' ('firm' can be similar in meaning to 'secure').
'not entirely' means to remove the last letter.
'firm' with its last letter removed is 'FIR'.
(Other definitions for fir that I've seen before include "Kind of cone-bearing tree" , "Tree for Irishmen" , "Pine tree for Gaelic men?" , "wood" , "Evergreen tree for Irish men?" .)