Measure young animal with it (5)
I believe the answer is:
cubit
'measure' is the definition.
(cubit is a kind of measure)
'young animal with it' is the wordplay.
'young animal' becomes 'cub' (eg a young bear).
'with' is a charade indicator (letters next to each other).
'cub'+'it'='CUBIT'
(Other definitions for cubit that I've seen before include "Measurement based on the length of the forearm" , "Biblical measure of length" , "Biblical length of approximately a forearm" , "Ancient unit of length based on the length of the forearm" , "Get the measure" .)