Article on tree enthrals radical trainspotter (6)
I believe the answer is:
anorak
'trainspotter' is the definition.
(I've seen this before)
'article on tree enthrals radical' is the wordplay.
'article' becomes 'an' (indefinite article).
'on' means one lot of letters go next to another.
'tree' becomes 'oak' (oak is a kind of tree).
'enthrals' indicates putting letters inside.
'radical' becomes 'r' (chemistry abbreviation).
'oak' going around 'r' is 'orak'.
'an'+'orak'='ANORAK'
(Other definitions for anorak that I've seen before include "Mikado's son (G&S)" , "good on most figures?" , "Raincoat" , "trainspotter" , "Casual outerwear" .)