Shapeless length in copper (6)
I believe the answer is:
blobby
'shapeless' is the definition.
(thesaurus)
'length in copper' is the wordplay.
'length' becomes 'l' (abbreviation in geometry).
'in' indicates putting letters inside.
'copper' becomes 'bobby' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'l' placed inside 'bobby' is 'BLOBBY'.
Can you help me to learn more?
(Other definitions for blobby that I've seen before include "with splashes of colour" , "Spotty" , "Indeterminate and roundish in shape" , "Indistinctly round" , "Indeterminately roundish" .)