Delayed using horse past its best? (2,4)
I believe the answer is:
on hold
'delayed' is the definition.
(on hold means delayed or postponed)
'using horse past its best?' is the wordplay.
'using' becomes 'on' (someone using drugs is 'on' them).
'horse' becomes 'h' (synonyms).
'past its best?' becomes 'old' (something old may be past its best days).
'on'+'h'+'old'='ON HOLD'
(Other definitions for on hold that I've seen before include "Temporarily set aside" , "set aside for future use" , "Waiting to be connected" , "waiting for reply" , "In a state of postponement" .)