Conductor giving brass little room (5,4)
I believe the answer is:
nerve cell
'conductor' is the definition.
(I've seen this before)
'giving brass little room' is the wordplay.
'giving' says to put letters next to each other.
'brass' becomes 'nerve' (synonyms).
'little room' becomes 'cell' (I've seen this before).
'nerve' next to 'cell' is 'NERVE-CELL'.
(Other definitions for nerve cell that I've seen before include "it provokes sensation" , "Neuron" , "excitable signaller" , "conductor" , "transmitter" .)