Small wood in an unkempt national park (9)
I believe the answer is:
snowdonia
'national park' is the definition.
(I've seen this before)
'small wood in an unkempt' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'unkempt' indicates anagramming the letters.
'wood'+'in'+'an'='woodinan'
'woodinan' with letters rearranged gives 'nowdonia'.
's'+'nowdonia'='SNOWDONIA'
(Other definitions for snowdonia that I've seen before include "Welsh mountain district" , "Welsh mountain region" , "Welsh mountain area" , "Britain's third national park, designated 1951" , "National park in Wales" .)