Solid mass next to tree (4)
I believe the answer is:
firm
'solid' is the definition.
(synonyms)
'mass next to tree' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'next to' says to put letters next to each other.
'tree' becomes 'fir' (fir tree is a kind of tree).
'm' put after 'fir' is 'FIRM'.
(Other definitions for firm that I've seen before include "Unyielding; company" , "Steady kind of business" , "Well-established - business" , "Unyielding business concern?" , "Not soft or yielding to pressure" .)