Trees, a mathematical number in lines (6)
I believe the answer is:
rowans
'trees' is the definition.
(rowan is a kind of tree)
'a mathematical number in lines' is the wordplay.
'a mathematical number' becomes 'an' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'lines' becomes 'rows' (row is a kind of line).
'an' going within 'rows' is 'ROWANS'.
Can you help me to learn more?
(Other definitions for rowans that I've seen before include "Mountain ashes" , "Mountain ash trees" .)