Imaginary line, moulds glued on it (9)
I believe the answer is:
longitude
'imaginary line' is the definition.
'longitude' can be an answer for 'line' (longitude is a kind of line). I am not certain of the 'imaginary' bit.
'moulds glued on it' is the wordplay.
'moulds' indicates anagramming the letters (I've seen 'mould' mean this).
'glued'+'on'+'it'='gluedonit'
'gluedonit' with letters rearranged gives 'LONGITUDE'.
Can you help me to learn more?
(Other definitions for longitude that I've seen before include "this helps define our position" , "Degree from Greenwich" , "Length" , "Distance in degrees" , "Word often paired with latitude" .)