Tree, by the start of May, should be solid (4)
I believe the answer is:
firm
'solid' is the definition.
(I know that solid can be written as firm)
'tree by the start of may' is the wordplay.
'tree' becomes 'fir' (fir is a kind of tree).
'by' says to put letters next to each other.
'the start of' suggests taking the first letters.
The initial letter of 'may' is 'm'.
'fir'+'m'='FIRM'
'should be' acts as a link.
(Other definitions for firm that I've seen before include "Steadfast" , "team of doctors in hospital" , "Unyielding business concern?" , "'Solid, unyielding (4)'" , "A steady business enterprise? (4)" .)