Exhausted, but steady now and then (4)
I believe the answer is:
used
'exhausted' is the definition.
(exhaust can mean to use up)
'but steady now and then' is the wordplay.
'now and then' means one should take alternating letters (letters taken on a regular basis, now and then).
'but'+'steady'='butsteady'
The alternating letters of 'butsteady' are 'USED'.
(Other definitions for used that I've seen before include "Dues (anag.)" , "No longer new" , "Hand-me-down" , "Utilised" , "exhausted" .)