Resin adds mass to timber (6)
I believe the answer is:
balsam
'resin' is the definition.
(balsam is a kind of resin)
'mass to timber' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'to' says to put letters next to each other (I've seen this in other clues).
'timber' becomes 'balsa' (I've seen this before).
'm' put after 'balsa' is 'BALSAM'.
'adds' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for balsam that I've seen before include "Fragrant resin used in medicine and perfumes" , "Resin-yielding tree" , "(Aromatic medicine from) shrub" , "Aromatic soothing medicament" , "Fragrant substance exuded by some trees" .)