Tree, a hawthorn almost blocking path (8)

'tree' is the definition.
(tamarack is a kind of tree)

'a hawthorn almost blocking path' is the wordplay.
'hawthorn' becomes 'may' (may is a kind of hawthorn).
'almost' means to remove the last letter (most of the word but not all of it).
'blocking' indicates putting letters inside.
'path' becomes 'track' (track is a kind of path**).
'a'+'may'='amay'
'amay' with its final letter removed is 'ama'.
'ama' going into 'track' is 'TAMARACK'.