Urn one's broken in cell (6)
I believe the answer is:
neuron
'cell' is the definition.
(neuron is a kind of cell)
'urn one's broken' is the wordplay.
'broken' indicates an anagram.
'urn'+'one'='urnone'
'urnone' anagrammed gives 'NEURON'.
'in' acts as a link.
(Other definitions for neuron that I've seen before include "Conductor" , "Transmitter of nerve impulses" , "Part of one's nervous system" , "Transmitting nerve cell" , "Cell that conducts nerve impulses" .)