Insect in tree climbing for cover (6)
I believe the answer is:
mantle
'cover' is the definition.
(mantling is a kind of covering)
'insect in tree climbing' is the wordplay.
'insect' becomes 'ant' (ant is a kind of insect).
'in' indicates putting letters inside.
'tree' becomes 'elm' (elm tree is a kind of tree**).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'elm' reversed gives 'mle'.
'ant' placed into 'mle' is 'MANTLE'.
'for' is the link.
(Other definitions for mantle that I've seen before include "Loose outer garment or covering" , "Layer below earth's crust" , "Cloak and shelf over the fireplace" , "Robe or gas-lamp fitting" , "Loose cloak, shawl" .)