A number rule the roost, to some extent (5)
I believe the answer is:
ether
'a number' is the definition.
(I've seen this before)
'rule the roost to some extent' is the wordplay.
'to some extent' indicates a hidden word.
'ETHER' is hidden within 'rule the roost'.
(Other definitions for ether that I've seen before include "Volatile organic liquid once used as a general anaesthetic" , "Gas - air - space" , "Colourless volatile highly inflammable liquid" , "Light liquid used as anaesthetic" , "Once widely-used anaesthetic" .)