Fellow arrives beside a large thicket of trees (9)
I believe the answer is:
chaparral
'thicket of trees' is the definition.
(term for an amount of vegetation or plants)
'fellow arrives beside a large' is the wordplay.
'fellow' becomes 'chap' (synonyms).
'arrives' becomes 'arr' (abbreviation used in transport).
'beside' is a charade indicator (letters next to each other).
'large' becomes 'l'.
'chap'+'arr'+'a'+'l'='CHAPARRAL'
(Other definitions for chaparral that I've seen before include "thicket" , "US growth area" .)