Carnivore's length added to value (5)
I believe the answer is:
ratel
'carnivore's' is the definition.
(ratel is a kind of carnivore)
'length added to value' is the wordplay.
'length' becomes 'l' (abbreviation in geometry).
'added to' means one lot of letters go next to another.
'value' becomes 'rate' (both can mean to esteem).
'l' after 'rate' is 'RATEL'.
(Other definitions for ratel that I've seen before include "Honey badger" , "one wanting honey" , "creature with sweet tooth" , "Badger-like creature" , "wild animal" .)