Curve in tree initially hidden (4)
I believe the answer is:
arch
'curve' is the definition.
(arch is a kind of curve)
'tree initially hidden' is the wordplay.
'tree' becomes 'arc' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'initially' indicates taking the first letters.
The first letter of 'hidden' is 'h'.
'arc'+'h'='ARCH'
'in' is the link.
Can you help me to learn more?
(Other definitions for arch that I've seen before include "Clever" , "Curve of the foot" , "Supporting structure" , "Structural curve" , "not to go straight?" .)