Resin in forest fruit, principally in chestnuts (6)
I believe the answer is:
mastic
'resin in forest' is the definition.
'mastic' can be an answer for 'resin' (mastic is a kind of resin). I'm not sure about the 'in forest' bit.
'fruit principally in chestnuts' is the wordplay.
'fruit' becomes 'mast' (mast is a kind of fruit**).
'principally' indicates taking the first letters.
The first letters of 'in chestnuts' is 'ic'.
'mast'+'ic'='MASTIC'
Can you help me to learn more?
(Other definitions for mastic that I've seen before include "Waterproof sealant" , "Resin used in varnishes" , "Gum from trees" , "liquor" , "Mouldable cement" .)