Instrument that accompanied mail, deployed on short trip ultimately (4,4)
I believe the answer is:
post horn
'instrument that accompanied mail' is the definition.
(instrument once used by to announce mail coaches)
'deployed on short trip ultimately' is the wordplay.
'deployed' indicates an anagram.
'ultimately' indicates one should take the final letters.
The last letter of 'trip' is 'p'.
'on'+'short'+'p'='onshortp'
'onshortp' with letters rearranged gives 'POST-HORN'.
(Other definitions for post horn that I've seen before include "Instrument once sounded on journey" , "Mail coach instrument" .)