Girl put up ornate tree on Circle Line (8)
I believe the answer is:
diameter
'circle line' is the definition.
(I've seen this in another clue)
'girl put up ornate tree' is the wordplay.
'girl' becomes 'maid' (I've seen this in other clues).
'put up' says the letters should be written in reverse.
'ornate' indicates an anagram (an unusual or complex form of the letters).
'maid' in reverse letter order is 'diam'.
'tree' anagrammed gives 'eter'.
'diam'+'eter'='DIAMETER'
'on' acts as a link.
(Other definitions for diameter that I've seen before include "Line that crosses the centre of a circle" , "Longest chord of a circle" , "It's equal to twice the radius" , "Paying back (of money)" , "Maximum distance across circle" .)